12-Month BAS Income Calculation: Step-by-Step 2026

Introduction

The 2026 lending environment for self-employed Australians continues to centre on the 12-month Business Activity Statement (BAS) as the primary income document for low-documentation home loans. Arrivau provides independent, fact-dense guidance without personal advice. In 2026, data-matching between the Australian Taxation Office and credit providers is expected to reach near-real‑time status under the Consumer Data Right expansion, making BAS accuracy more critical than ever. This article sets out a step‑by‑step method for calculating assessable income from a 12‑month BAS series, incorporating ATO small business benchmarks, APRA prudential limits, and common lender overlays.

What a BAS Is and Its Role in Low Doc Lending

A Business Activity Statement reports Goods and Services Tax (GST) obligations, including total sales (label G1), GST collected, and GST paid. For GST-registered sole traders, partnerships, and companies, the ATO issues monthly or quarterly BAS forms. Low doc lenders rely on these statements because self-employed borrowers rarely produce conventional PAYG payslips. The Australian Prudential Regulation Authority (APRA) Prudential Standard APS 220 requires lenders to verify income using reliable, third‑party data. BAS forms, lodged with the ATO, satisfy this requirement when a full 12-month set is presented.

From 2024, APRA’s enhanced guidance on low doc lending pushed lenders to reject photocopies alone, moving toward electronic ATO portal verification. In 2026, most lenders require a direct ATO income statement feed via the Consumer Data Right (CDR) framework, referenced in the Treasury CDR roadmap. Consequently, the borrower’s declared BAS figures must match the ATO’s record precisely.

Collecting and Preparing 12 Months of BAS Statements

Borrowers must assemble the latest four quarterly BAS (or twelve monthly BAS) covering a continuous 12‑month period ending no earlier than six weeks before the application date. The ATO’s Business Activity Statement guidelines detail how to access historical statements via ATO Online Services or a registered tax agent.

Essential data fields for income calculation:

• Label G1: Total sales (including GST)
• Label G2: Export sales (GST‑free)
• Label G3: Other GST‑free sales
• Label 1A: GST on sales

Lenders typically use G1 as the top-line figure. Exported and GST‑free amounts (G2, G3) may be added back depending on the lender’s credit policy. For non‑GST registered businesses, the BAS is not required; a lender would instead request an accountant’s letter and 12 months of business bank statements. The examples below assume GST registration.

Each quarter’s G1 figure is taken directly from the ATO‑lodged BAS. A borrower who runs a café reports Q1: $33,000, Q2: $44,000, Q3: $38,500, Q4: $49,500 (all inclusive of GST). The raw sum is $165,000.

Step‑by‑Step Calculation Methodology

Lenders follow a standardised sequence, largely unchanged in 2026 except for automated sourcing from the ATO. The five‑stage process is:

1. Sum G1 totals across four quarterly BAS (or twelve monthly).
2. Remove GST to derive tax‑free gross revenue. For businesses where all sales are GST‑inclusive, divide the sum by 1.1. If a portion of sales is GST‑free (G2 + G3 > 0), the calculation becomes: (G1 / 1.1) + (G2 + G3). In the café example, with no GST‑free sales, $165,000 ÷ 1.1 = $150,000.
3. Apply an expense ratio using the ATO’s small business benchmarks. The benchmarks provide average cost‑of‑sales to turnover ratios for over 100 industries. For a café (ANZSIC code H4511), the benchmark cost of sales is 52%, implying a gross profit ratio of 48%. The lender uses this ratio to estimate gross profit: $150,000 × 0.48 = $72,000. If the borrower can demonstrate a higher net profit via an accountant’s statement, some lenders accept a declared profit ratio of up to 65%, but must reconcile with the BAS total.
4. Determine assessable income. Most lenders take a conservative approach, using 80% of the estimated gross profit (or 100% of verified net profit) for serviceability. For the café, 80% of $72,000 = $57,600 per annum.
5. Apply debt-to-income (DTI) and LVR constraints. APRA’s non‑public DTI hurdle of 6 times gross income applies to many lenders. For the $57,600 assessable income, maximum total debt is $345,600. If the borrower already holds other debts, available mortgage capacity reduces. Loan‑to‑value ratio (LVR) for low doc loans typically caps at 60% to 80% of the property value, depending on the loan size and credit score. According to the RBA’s Financial Stability Review – October 2024, low doc loans remain less than 2% of new housing credit, with average LVR at 63%.

2026 Policy Adjustments Affecting BAS Calculations

The Consumer Data Right, administered by Treasury, enables lenders to retrieve ATO‑reported figures directly. Starting 1 January 2026, lenders accredited under open banking must use CDR‑sourced BAS data as the primary verification, eliminating paper‑based assessment for most applications. This change means:

• Calculations are performed automatically by lender platforms, leaving no room for manual adjustment of G1 figures.
• The ATO benchmark profit ratio is algorithmically applied; if a borrower claims a higher ratio, the lender must request a full tax return as a supporting document, effectively moving the application to a full‑doc track.
• Lenders are required under APRA’s APS 220 to update benchmark ratios each year from the ATO’s published data. For 2026, the benchmarks released in March 2025 are effective.

Borrowers who have amended past BAS after original lodgement should be aware the ATO’s record may differ from their paper copies. Any discrepancy can delay or derail an application.

Worked Example with Detailed Figures

A self-employed electrician (ANZSIC E3222, ‘Electrical Services’) has a strong trading history and registers for GST on a quarterly basis. The four most recent BAS G1 totals are:

• Jul–Sep 2025: $55,000
• Oct–Dec 2025: $68,750
• Jan–Mar 2026: $49,500
• Apr–Jun 2026: $60,500 Total G1: $233,750. No GST‑free sales.

Gross revenue (GST removed): $233,750 ÷ 1.1 = $212,500.

The ATO benchmark for Electrical Services shows an average cost of sales of 38% (i.e. gross profit ratio 62%). Estimated gross profit: $212,500 × 0.62 = $131,750.

Lender assesses 80% of this figure: $105,400 p.a. assessed income.

Applying a DTI cap of 6, maximum total debt serviceable is $105,400 × 6 = $632,400. Assume the borrower has an existing car loan with a limit of $30,000; remaining capacity is $602,400.

For an 80% LVR low doc loan (maximum for this risk grade), the maximum property value supported is $753,000 if no other debts (80% of $753,000 = $602,400). However, at a stress‑tested assessment rate of 9.5% (incorporating a 3% buffer above the current average owner‑occupier variable rate of 6.5%, per APRA’s Mortgage Serviceability Guidance), the monthly repayment on a $602,400 loan over 30 years is approximately $5,070. This consumes 57.7% of the assessed annual income of $105,400, marginally below the 60% serviceability ceiling commonly applied. Thus, the loan may be approved, but any increase in existing liabilities would tip the ratio.

Common Pitfalls and Verification Requirements

The ATO data‑sharing environment has reduced fraud but introduced new friction points. Five common issues:

• Timing gaps: Borrowers who lodge a later amendment for a missing quarter will find the ATO record out of sync. Lenders require an unbroken 12‑month history.
• Inconsistent reporting: Round‑sum G1 entries that suggest estimation rather than actual sales trigger heightened scrutiny.
• Industry mismatch: The ANZSIC code on the BAS must match the business activity. A borrower reporting café numbers but whose ABN registration shows ‘management consultant’ will be challenged.
• GST‑free overlays: Lenders differ on adding back G2/G3. Arrivau data indicates only 40% of low doc lenders include GST‑free sales in the top‑line unless the borrower provides a detailed profit and loss statement.
• Accumulated ATO debt: Lenders now check the tax‑agent portal for overdue BAS liabilities. A debt threshold above $10,000 may disqualify the applicant under APRA’s responsible lending expectations.

Every low doc application in 2026 must be accompanied by a signed accountant’s declaration and, where the lender is an ADI, a CDR consent form to pull BAS data. Arrivau advises retaining all BAS originals, bank statements showing GST outflows, and quarterly profit summaries to support any manual verification.

Conclusion

The 12‑month BAS calculation remains the backbone of low doc loan underwriting, but 2026’s technological and regulatory changes demand forensic accuracy from self‑employed borrowers. By summing G1 labels, stripping GST, applying the relevant ATO benchmark, and multiplying by the lender’s income‑acceptance percentage (typically 80%), an applicant can estimate their assessable income. The worked example shows a licensed electrician with $212,500 gross revenue yielding an assessed $105,400, supporting up to $602,400 of debt within a DTI of 6. All rates, benchmarks, and LVR caps cited are sourced from the ATO, APRA, and RBA. Prospective borrowers must verify lender‑specific overlays and ensure their BAS data aligns with the ATO’s electronic record before applying.

Information only, not personal financial advice. Consult a licensed mortgage broker.