Mortgage Repayments in Australia: How They Actually Work (2026)

Three numbers determine your monthly mortgage repayment: the loan amount, the interest rate, and the loan term. Everything else (fortnightly vs monthly, offset accounts, redraw, fixed splits) is layered on top of that core calculation. Once you understand the core, you can spot which "save thousands!" tricks are real and which are just calendar maths dressed up as a feature.

This guide walks through the maths in plain English, with numbers from a typical Adelaide buyer in 2026. Want to test your own scenario directly? The Mortgage Calculator handles weekly, fortnightly, and monthly P&I or interest-only, with extra-repayment savings shown over the life of the loan.

P&I vs interest-only

Principal and Interest (P&I) is the default. Each repayment covers the interest accrued that month plus a chunk of principal. Over the term, the proportion paid as principal grows and the proportion paid as interest shrinks. At the end of the term, the loan is fully repaid.

Interest-Only (IO) means you pay only the interest. The loan balance stays the same. Repayments are lower because you're paying less per month, but you make no progress on the actual debt. Most lenders cap IO at 5 years before requiring conversion to P&I.

IO is almost always wrong for an owner-occupier. The lower monthly payment is offset by:

• No equity being built across the IO term, so at the end you still owe the original amount.
• The eventual switch to P&I compresses the principal repayment into a shorter remaining term, which spikes the monthly payment.
• You're not eligible for negative-gearing benefits because you live in the property.

IO can make sense for investors who want maximum tax deductibility, with the explicit understanding that they're not paying down debt. For a home you live in, P&I from day one is almost always the better answer.

The amortisation curve, and why early extra repayments matter so much

On a $700,000 loan at 6.50 percent over 30 years, the monthly P&I repayment is roughly $4,425. In month one, $3,792 of that goes to interest and just $633 to principal. By year 15 it's roughly even. By year 25, most of the payment is principal.

This explains why extra repayments early in the loan have an outsized effect. An extra $1,000 paid in month 6 prevents 30 years of compounding interest on that $1,000. The same $1,000 paid in month 300 only prevents 5 years of compounding. The earlier you pay extra, the more interest you save.

A practical illustration of the compounding effect:

Strategy on $700,000 / 6.50% / 30-year loan	Years to payoff	Total interest paid
Standard monthly P&I	30.0	$893,000
Fortnightly P&I (same dollars per year)	26.3	$748,000
Monthly P&I + $100/week extra	22.8	$650,000
Monthly P&I + $250/week extra	18.1	$486,000

These are approximations; your actual numbers depend on the exact rate path and rounding. But the order-of-magnitude is real: an extra $100 a week from the start of a 30-year mortgage saves you roughly a quarter of a million dollars in interest.

The fortnightly vs monthly "trick"

The standard pitch: "switch to fortnightly repayments and pay off your mortgage 4 years sooner!"

The catch: it only works if your fortnightly amount is exactly half your monthly amount. There are 26 fortnights in a year, so 26 fortnightly payments at half the monthly amount equal 13 monthly payments per year, not 12. The "trick" is that you're making the equivalent of one extra month's payment per year.

You get the same result by setting up monthly P&I and adding one extra payment per year directly. Or by adding 1/12 of a monthly payment to every month. There's no magic in the fortnightly calendar; it's just a way of disguising the extra payment as routine.

That said, the disguise works. Most people stick to fortnightly more reliably than they would stick to "monthly plus an extra payment". Behavioural finance counts.

Offset accounts vs redraw

Both reduce interest by offsetting your loan balance with cash you happen to have. The difference is in how the money is held and how easy it is to access.

• Offset is a separate transaction account linked to your loan. The balance reduces the interest calculation but the cash remains yours, accessible at any time. Most variable-rate loans offer an offset; fixed-rate loans typically do not.
• Redraw is access to extra repayments you've already made. You pay extra into the loan, the balance drops, and you can later "redraw" some of that extra back out if needed. Tax-wise, redraw can complicate things if the loan was used for investment.

For an owner-occupier, offset and redraw produce the same interest saving. Offset is more flexible and tax-cleaner; redraw is simpler. For an investor, offset is almost always preferable because withdrawing from an offset doesn't change the deductibility of the loan, whereas redraw does.

Fixed vs variable

Fixed rates give certainty over a 1 to 5 year period, usually at a slightly higher rate than the current variable. If rates rise during your fixed period, you win. If rates fall, you've locked yourself out of the lower rate (and breaking the fix can be expensive).

Variable rates move with the RBA's cash rate and the bank's funding cost. They go up and down. If you can absorb a $300 to $500 monthly swing in your repayments without stress, variable is usually fine. If a sudden rate rise would force a lifestyle change, fixing part of the loan is reasonable insurance.

The "split loan" approach (fix half, leave half variable) gets you partial certainty and partial flexibility. There's no single right answer; it depends on your view of where rates are heading and your tolerance for surprises.

Frequently asked questions

The bottom line

Mortgages compound. A small change to the rate, the term, or the extra repayments can shift the lifetime cost by hundreds of thousands of dollars. The earlier you make extra repayments, the more they save. The further you can get from the bank's standard variable rate (by negotiating, refinancing, or splitting), the better. Use the Mortgage Calculator with your actual loan number to see how much each lever is really worth.