An adjustable-rate mortgage (ARM) is the American term for a home loan whose interest rate moves with market conditions over the life of the loan. In Australia the closest equivalent is the standard variable-rate home loan: the rate starts at one number, then rises and falls as your lender responds to funding costs and Reserve Bank decisions, which is why most Australian brokers know the concept simply as a variable rate.
This guide explains how the moving-rate structure works, what it borrows from the American ARM and where Australian products differ, then how to work out whether a variable structure suits a given borrower.
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The Australian versions of an adjustable rate
Australian home loans come in three structures. A pure variable loan adjusts continuously with lender pricing decisions. A fixed-period loan locks a rate for one to five years and then reverts to a variable rate, which behaves like the American hybrid ARM without the formal reset dates. A split loan runs part fixed and part variable side by side.
The trade-off matches the classic ARM logic: variable and introductory rates usually start below the equivalent fixed-rate mortgage, but the borrower carries the risk that rates rise later. Many lenders also offer introductory or honeymoon discounts that expire after one or two years, so always compare using the revert rate rather than the headline discount.
Features worth checking
American ARMs carry formal machinery such as index benchmarks, margins, periodic caps and lifetime caps. Australian variable loans have none of that contract scaffolding; the rate simply moves whenever the lender reprices, with no cap limiting the move. That makes four checks more useful than any formula:
- Revert rate: confirm what an introductory discount rolls onto after the promo period ends.
- Rate-change history: check how often and by how much the lender has moved this product in recent years.
- Flexibility features: confirm offset access, redraw and extra-repayment terms survive on the discounted product.
- Exit costs: check break fees on any fixed portion and discharge costs before committing.
Negative amortisation: mostly an import problem
Negative amortisation, where unpaid interest is added to the principal and the debt grows, was a notorious feature of American option ARMs. Standard Australian principal-and-interest loans cannot do this because each payment covers interest plus some principal by contract. The risk only appears locally through extended interest-only periods or hard redraw drawdowns, so treat long interest-only terms as the place to look when stress-testing a client’s loan.
Who sets the rate
The Reserve Bank cash rate target sat at 4.35% from August 2026, but lenders set their own variable rates and are not obliged to move in step with RBA changes, so two variable products can drift apart over time. On the lending side, APRA requires authorised deposit-taking institutions to test new borrowers at their loan’s interest rate plus a serviceability buffer of at least three percentage points, deliberately ignoring discounted introductory rates, while non-bank lenders fall outside that ADI requirement.
Choosing the structure
Suit a variable structure to borrowers who can absorb a rate rise of several percentage points, expect to sell or refinance within a few years, or value offset and extra repayments over payment certainty. Suit a fixed or split structure to borrowers on tight budgets for whom payment shock would force hardship. Either way, Budgeting for a rise before settling the structure beats discovering the limit after the first increase.
Your next step
Take your next variable-rate client scenario and model the repayment at today’s rate plus three percentage points. If the result still fits their budget comfortably, the variable structure stands; if not, price a fixed or split alternative before lodging.
