Every mortgage repayment in Australia splits two ways: the principal, which is the amount borrowed, plus the interest, which is the lender’s charge for lending it. How those two components move over the life of a loan depends on the repayment structure you and your client choose, and that choice changes the total cost of the loan more than most borrowers expect.
This guide explains how principal and interest interact, compares the two main repayment structures used by Australian lenders, and sets out what a broker should check before recommending either one.
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How principal and interest work together
The principal starts as the purchase price minus the deposit, plus any costs capitalised into the loan. Interest is charged on the outstanding balance, calculated daily by most lenders and charged monthly. Early in a principal and interest loan, interest takes the largest share of each repayment because the balance is at its highest. As the balance falls, the interest component shrinks and more of each payment reduces the debt.
This is why the loan term matters so much. A 25-year term and a 30-year term can carry similar rates while producing very different total interest figures, because the longer term keeps the balance higher for longer. Showing clients an amortisation figure for both terms is often the fastest way to make the trade-off concrete.
Principal and interest versus interest-only
A principal and interest (P&I) repayment covers the accrued interest plus a slice of the debt, so the balance falls every month and equity builds steadily. Most owner-occupier loans in Australia use this structure.
The second structure, Interest-only loans, keeps the balance flat for the interest-only period, commonly up to five years for owner-occupiers and around ten for investors, although each lender sets its own limits. The appeal is cash flow: the minimum payment is smaller. The trade-off is that no debt is repaid during that period, and when it ends the repayments recalculate over the remaining years, often producing a sharp step-up that clients should see modelled before they commit.
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The long-run cost difference
Interest-only costs more overall whenever the rate is above zero, simply because the balance stays higher for longer. That does not make it wrong. Investors sometimes accept the extra cost to maximise deductible borrowings or free up cash elsewhere, and some owner-occupiers use a short interest-only period to survive parental leave or a business ramp-up. The test is whether the client has a stated plan for converting to P&I and can service the higher repayment that follows.
Choosing a structure for a client
Work through four questions. What is the property purpose, since deductibility and APRA servicing rules differ between owner-occupied and investment lending? How tight is the budget today, and how certain is future income? When would the client realistically sell or repay? And how will the exit from any interest-only period be managed? Aligning the resulting choice with documented goals also strengthens the responsible-lending file if the client’s situation later changes.
The client’s credit score, deposit and existing debts shape pricing on both structures, so it pays to tidy those before applying rather than after. A mortgage broker can then compare lenders on more than headline rate, including how each prices interest-only terms and what reset servicing looks like.
Where deposit size triggers insurance
Mortgage insurance, usually called LMI, typically applies when the deposit is below 20 per cent of the property value. It protects the lender, not the borrower, but it can let a well-qualified client buy sooner. Because premiums scale with loan size and lender, comparing insurers through your aggregator can save real money, and some professions qualify for waivers.
Reviewing and restructuring later
Circumstances change, and so do rates. A structured annual review catches interest-only periods approaching their end, fixed terms rolling to revert rates, and equity that has grown enough to reprice. Refinancing can lower the rate or extend breathing room, though break costs on fixed loans and application fees need weighing first. For clients juggling competing goals, bring in their accountant or planner; guidance on budgeting and tax sits outside credit advice.
A check worth running before you recommend
For the next client conversation, run their proposed loan through an amortisation comparison at both P&I and interest-only, including the post-interest-only repayment. A client who sees the step-up coming rarely disputes it later, and the file shows the recommendation was tested.
