How Australian Banks Make Money From Mortgages: The Real Economics

Most commentary about bank profits misses the mechanics entirely. Banks are not profitable because they charge high interest rates — Australian mortgage rates are competitive by international standards. They are profitable because of the interaction between funding cost, regulatory capital, loss rates and volume. This article works through that arithmetic step by step, because it is the single most useful thing to understand about Australian financial services.

What exactly is net interest margin?

Net interest income divided by average interest-earning assets. If a bank earns 5.5% on its loans and pays 3.5% on the mix of deposits, wholesale debt and equity that funds them, its net interest margin is roughly 2%. CBA reported 2.08% in FY2025, a 9 basis point improvement on the prior year.

The composition matters more than the number. Margin improves when a bank holds more low-cost transaction deposits, when the return on the capital it holds rises with interest rates, and when hedging on its replicating portfolio is favourable. It deteriorates when depositors shift into higher-rate term deposits and savings accounts, or when mortgage competition forces front-book discounting.

This is why the mortgage price war and the deposit war are the same war. Every major bank wants to grow lending without paying up for deposits, and all four are trying simultaneously in a market that is not growing fast enough to accommodate them. The result is compressed margins even in a rising rate environment.

Why do mortgages produce such high returns on equity?

Because of regulatory risk weights. Capital rules require banks to hold equity in proportion to risk-weighted assets rather than raw assets, and Australian residential mortgages carry low risk weights — a reflection of historically very low loss rates. A bank might hold only a few cents of equity against a dollar of well-secured home lending.

Run the arithmetic and the result is unavoidable. Take a large loan book, apply a 2% margin, subtract operating costs and small loan losses, and divide the remaining profit by a small equity base. That is how CBA generates a 13.7% return on equity while holding a 12.3% CET1 ratio, which is conservative by global standards.

The corollary is that capital requirements, not interest rates, are the binding constraint on Australian bank returns. Every time APRA raises risk weights or capital benchmarks, returns fall mechanically regardless of what the bank does operationally. This is also why the majors’ internal ratings-based models — which produce lower risk weights than the standardised approach smaller banks use — are such a persistent competitive issue.

Why are loss rates so low in Australia?

Several reinforcing factors. Australian home loans are full-recourse, meaning a lender can pursue a borrower’s other assets and income after repossession, which removes the strategic default dynamic seen in parts of the United States. Lenders mortgage insurance covers higher loan-to-value lending. And the labour market has been strong for most of the period during which the mortgage book was built.

Structural features also help. A large share of Australian borrowers hold offset accounts and are ahead on repayments, providing a buffer against rate increases. Banks are required to assess new borrowers against a serviceability buffer above the actual rate, which builds headroom into the loan at origination.

The caveat is that low historical losses are not a law of nature. The Australian mortgage book has never been tested by a severe, sustained unemployment shock combined with falling house prices. Every risk model in the country is calibrated on data from a period in which that combination did not occur.

How do brokers change the economics?

They commoditise the product and shift power to whoever is cheapest this week. Broker-originated loans now make up the majority of new lending at several majors — roughly 67.5% at Westpac in 2025, up from 52% in 2023, and about 67% at ANZ — and they come with upfront and trail commissions that reduce the lifetime profitability of each loan.

NAB has quantified the difference, saying proprietary home lending returns 20–30% more than broker-originated business, and has pushed its broker share down toward 59%. All four majors are now hiring branch and mobile lenders again after a decade of reducing them, which is an expensive reversal.

For borrowers the effect is positive. Brokers make rate comparison easy, which is precisely why banks dislike them, and it is part of why ASIC has been able to document a persistent gap between what new and existing customers pay — typically 0.30% to 0.50% at the majors. If you have held the same mortgage for several years without renegotiating, that gap is being paid by you.

What happens to bank earnings when housing slows?

Three things compress at once, which is why analysts treat a housing slowdown as a compound rather than linear risk. Volume growth slows, because a mortgage book only grows when new lending exceeds repayments. Margin compresses, because banks compete harder for a smaller pool of borrowers. And loss provisions rise, because arrears follow the cycle.

The macro backdrop in 2026 has made this concrete. After cutting through 2025, the Reserve Bank raised the cash rate three times to reach 4.35% by May 2026, adding roughly A$272 a month to a A$600,000 variable-rate mortgage compared with 2025 levels. Analysts including Morgan Stanley have started describing the current period as the potential end of Australia’s thirty-year housing super-cycle.

The banks are not passive in this. NAB has been growing deposits and lending at around 9% annualised, ANZ has restored mortgage growth to system levels through a productivity programme, and CBA has continued gaining investor mortgage share. Well-run lenders manage downturns rather than simply absorbing them — but a structural slowdown in credit growth would still reset the earnings base for the whole sector. See our profiles of CBA and the Big Four structure for how each bank is positioned.

How does household debt shape the whole system?

Australian households carry one of the highest debt-to-income ratios in the developed world, built up over three decades of rising house prices, falling interest rates and steady credit expansion. That aggregate figure is the reason monetary policy transmits so powerfully in Australia: a change in the cash rate reaches household budgets faster and harder than in economies where fixed-rate mortgages dominate.

The variable-rate structure is the key mechanical difference. In the United States, most borrowers hold thirty-year fixed-rate mortgages, so rate rises affect only new borrowers. In Australia, most loans are variable or fixed for only two to three years, so a rate increase flows through to the majority of borrowers within months. That makes the Reserve Bank’s job easier and household finances more exposed.

For banks, high household leverage is both the source of the earnings base and the concentration of the risk. The same balance sheets that generate reliable interest income in a stable economy are the transmission mechanism for a shock if unemployment rises materially. This is why APRA’s macroprudential tools focus on the flow of new lending — serviceability buffers, limits on high debt-to-income lending — rather than on the existing stock, which cannot be unwound.

What tools does APRA use to limit mortgage risk?

Macroprudential policy, which targets the flow of new lending rather than the interest rate. The primary tool is the serviceability buffer, which requires lenders to assess borrowers at a rate meaningfully above the one actually offered, building repayment headroom into every new loan at origination.

APRA has also used direct limits when specific risks accumulated: caps on investor credit growth and on the share of interest-only lending were imposed in the mid-2010s and later removed once the composition of new lending improved. More recently the focus has been on the share of loans written at high debt-to-income multiples, which is monitored bank by bank.

The design logic is that the existing stock of mortgages cannot be changed, only the quality of what is added to it. Every quarter of restrained new lending improves the average quality of the total book slightly. It is a slow instrument, which is why regulators prefer to tighten early rather than react to arrears data that arrives eighteen months after the lending decision that caused it.

For borrowers, the practical implication of everything above is simple. The margin a bank earns on your loan is a function of its funding cost and its capital treatment, neither of which you can influence, and your negotiating position, which you can. A single phone call asking your lender to match its own advertised new-customer rate costs nothing and, given the documented gap between front-book and back-book pricing, succeeds often enough to be worth making annually.

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