Calculate interest on overdue progress payments at the correct rate and for the correct period — from the due date through to the date of actual payment — for QLD, NSW, VIC and SA.
The applicable interest rate depends on what the contract says and what the SOP legislation in your state provides. Sopal applies the correct rule for your jurisdiction so you are not guessing.
Interest under the security-of-payment legislation runs from the day the payment became due under the Act — not from the date of a judgment or adjudication determination. Getting that start date right matters.
Use the due-date calculator to confirm exactly when the payment fell due, then feed that date straight into the interest calculator to build the full overdue-payment picture.
Tell Sopal the jurisdiction, the claimed amount, when payment fell due, and when (or whether) payment was actually made. The calculator returns the total interest accrued and a day-by-day breakdown you can copy straight into a submission or letter of demand.
Each of the four state security-of-payment statutes that Sopal covers — Queensland's Building Industry Fairness (Security of Payment) Act 2017, the New South Wales Building and Construction Industry Security of Payment Act 1999, the Victorian Building and Construction Industry Security of Payment Act 2002, and the South Australian Building and Construction Industry Security of Payment Act 2009 — includes a provision entitling a claimant to interest where a progress payment is not made by the due date.
The general principle across these statutes is consistent: if a respondent fails to pay the full amount of a progress payment by the time it becomes due and payable, the unpaid amount carries interest from the due date. The rate and the precise mechanism for calculating that interest differ across jurisdictions, so it is important not to assume that what applies in one state applies everywhere.
In most jurisdictions the interest rate is determined in the following order of priority:
Sopal applies whichever rate is appropriate based on your inputs. If you have a contract rate that differs from the statutory default, you can enter it manually. Where the contract is silent, the calculator uses the applicable statutory or regulatory rate for the relevant jurisdiction.
Interest runs from the date the payment became due and payable under the Act — not from the date an adjudication determination is made, not from the date the claimant sends a follow-up letter, and not from some other milestone in the dispute. Correctly identifying the due date is therefore critical to getting the interest calculation right.
Each jurisdiction sets its own timeframes for when a payment becomes due. Those timeframes can be affected by what the contract says (up to the maximum the Act allows), the type of construction contract, and whether any legislative amendments have changed the default periods since the contract was entered into. If you are not sure exactly when the payment fell due, use Sopal's due-date calculator to work that out first, then bring the date across to the interest calculator.
The SOP legislation generally provides for simple interest — that is, interest calculated on the principal unpaid amount only, not on accumulated interest. This is different from compound interest, where interest accrues on previous interest. Sopal's calculator uses simple interest unless you specifically configure otherwise, in line with the standard statutory approach.
Where a respondent makes a partial payment — paying some but not all of the overdue amount — interest continues to accrue on the remaining unpaid balance from the original due date. It does not reset or restart. Sopal handles partial payment scenarios: you can record the amount and date of any partial payment, and the calculator will reflect the correct interest accrual on the reducing balance.
Interest on an overdue progress payment is a legitimate component of an adjudication application. Adjudicators in Queensland and other states regularly deal with interest claims as part of the overall determination. A clear, day-by-day interest schedule — showing the unpaid amount, the rate applied, and the daily accrual — is far more persuasive than a bare dollar figure with no working. Sopal's output is designed to be dropped directly into an adjudication application or supporting submission.
For access to Queensland adjudication decisions where interest was at issue — to understand how adjudicators have approached contested rate questions — see Sopal Research, which covers 7,300+ Queensland decisions with full-text search.
Payment disputes rarely involve a single clean overdue amount. Sopal's interest calculator handles partial payments, multiple periods, and open-ended running calculations where final payment has not yet been made.
While the core principle — interest runs from the due date on unpaid progress payments — is common to all four jurisdictions, the detail varies. The table below summarises the key differences at a high level. Always confirm the current provisions of the relevant Act and any applicable regulations before relying on a rate, as these can change and Sopal's in-app calculator reflects the current rules at the time of your calculation.
| Jurisdiction | Statute | Rate basis | Notes |
|---|---|---|---|
| Queensland | Building Industry Fairness (Security of Payment) Act 2017 (BIF Act) | Contract rate, or prescribed rate if contract silent | Prescribed rate set by regulation. Also relevant to adjudication amounts unpaid after determination. |
| New South Wales | Building and Construction Industry Security of Payment Act 1999 | Contract rate, or prescribed rate if contract silent | Prescribed rate determined by reference to the relevant regulations. NSW has been subject to amendments; confirm current provisions. |
| Victoria | Building and Construction Industry Security of Payment Act 2002 | Contract rate, or prescribed rate if contract silent | The Victorian Act has undergone significant reform. Verify the current interest provisions and applicable regulations. |
| South Australia | Building and Construction Industry Security of Payment Act 2009 | Contract rate, or prescribed rate if contract silent | SA was later to adopt SOP legislation than the east coast states. Confirm the current prescribed rate with the relevant Act and regulations. |
If your contract specifies an interest rate for late payment of progress claims, that rate generally prevails. If it does not, the statutory prescribed rate applies. Sopal lets you toggle between the two so you can calculate under either scenario.
When a head contractor is late paying your progress claim, you are likely entitled to interest from the due date. Sopal tells you how much, so you can include the correct figure in your adjudication application or letter of demand — and not leave money on the table.
If a principal is slow to pay, the same rules apply to your own progress claims. And if you need to assess a subcontractor's interest claim against you, Sopal lets you verify their calculation quickly before you respond.
Assessing or certifying a payment? Being able to quantify interest accurately — including the correct start date and rate — is part of getting the certificate right and avoiding disputes about the figure later.
Understanding your potential interest exposure on disputed or delayed payments helps you make better decisions about whether and when to pay, and how to assess settlement offers.
The interest calculator and the due-date calculator are designed to work together. The due-date calculator establishes exactly when a progress payment became due and payable under the relevant Act — taking into account the reference date, the type of contract, what the contract says about payment periods, and the statutory maximum. That date feeds directly into the interest calculator as the start of the interest period.
Using them in sequence removes the two biggest sources of error in DIY interest calculations: using the wrong due date, and applying the wrong rate. Together, they give you a defensible, jurisdiction-correct figure you can put in front of an adjudicator or use to anchor settlement discussions.
Both tools are part of Sopal Projects, which also includes the payment-claim review, payment-claim builder, payment-schedule review, and payment-schedule builder. If you want access to the full Queensland adjudication decision database and AI case-law research alongside the calculator tools, Sopal Plus includes both products on one plan.
Sopal first asks whether your contract specifies an interest rate for overdue progress payments. If it does, you can enter that rate and the calculator will use it. If the contract is silent — which is common — the calculator applies the prescribed statutory rate for the jurisdiction you have selected. The prescribed rates differ between QLD, NSW, VIC and SA and are set by or under the relevant Act or its regulations. Sopal keeps these rates current; the in-app calculator will reflect the applicable rate at the time you run the calculation.
Interest runs from the date the progress payment became due and payable under the SOP legislation — not from the date you lodge an adjudication application, not from the date a determination is made, and not from the date you send a formal demand. The due date is determined by the contract (subject to any statutory cap on payment periods) and the relevant Act. If you are not certain of the exact due date, use Sopal's due-date calculator before running the interest calculation.
Sopal calculates simple interest by default, which is the approach taken by the SOP legislation across the jurisdictions it covers. Simple interest accrues on the principal unpaid amount only, not on accumulated interest. If your contract specifies compound interest for some reason, you would need to account for that separately — but in the vast majority of construction payment scenarios, simple interest is the correct approach.
Sopal handles partial payments. If the respondent has paid some but not all of the overdue amount, you can record the partial payment — the amount and the date — and the calculator will apply interest on the full outstanding balance from the original due date up to the partial payment date, then continue accruing interest on the reduced balance from that point forward. Interest does not restart from the partial payment date; it continues on the remaining unpaid amount.
Yes. Where an adjudicator makes a determination and the respondent fails to pay the adjudicated amount, the SOP legislation in most jurisdictions provides for interest on the unpaid determination amount as well. The mechanics are similar — interest runs from the date the determination amount was due to be paid. You can use Sopal's interest calculator for this scenario; just enter the adjudicated amount and the date by which it was required to be paid as your start date.
Yes — and you should, if you are entitled to it. Interest on an overdue progress payment is a legitimate head of claim in an adjudication application. Adjudicators regularly award interest as part of the adjudicated amount. Providing a clear, itemised interest schedule (showing the unpaid principal, the rate applied, and the daily and total interest accrued) strengthens your application and makes it easier for the adjudicator to allow the claim without having to do their own calculation.
Yes. Sopal's interest calculator covers QLD, NSW, VIC and SA — the four jurisdictions where Sopal Projects operates. Simply select the relevant state and the calculator applies the correct rate and rules. Note that the adjudication decision database in Sopal Research is Queensland-only (7,300+ decisions), but the calculator and other Projects tools are available across all four states.
No. Sopal is general information software, not legal advice. The interest calculator is a tool to help you understand and quantify what the legislation provides — it does not tell you whether you have a valid payment claim, whether your claim complies with all requirements of the Act, or how an adjudicator will decide any particular issue. If you have a complex dispute or are unsure about your position, get advice from a construction lawyer. For legal professionals who want access to case law and adjudication decisions to support their advice, see Sopal Research.
Sopal Projects includes the interest calculator, the due-date calculator, payment-claim and payment-schedule tools — for QLD, NSW, VIC and SA. 14-day free trial, no commitment.
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