Every dollar sitting against your mortgage — whether in an offset account or paid in as an extra repayment — saves you exactly the same interest: your loan rate, tax-free, guaranteed. At 6%, that's a better after-tax return than a savings account earning 5% (which loses a third to tax at typical marginal rates) and it carries zero risk.
So the choice between offsetting and repaying extra is not about the rate of return. It's about four second-order things that people discover too late.
First, the shared maths
Take a $600,000 loan at 6% over 30 years. The monthly repayment is about $3,597, and in month one, $3,000 of that is interest.
Park $20,000 in an offset account and the loan behaves as if it were $580,000: interest drops by $100 in the first month, about $1,200 in the first year. Pay the same $20,000 in as an extra repayment and the interest saving is identical. In both cases your repayment stays the same, so the saved interest quietly accelerates your principal — which is why a single early lump sum can remove years from a 30-year loan. Run your own figures in the offset calculator or the extra repayments calculator; the interest saved will match to the dollar.
The differences start after the money goes in.
Difference one: access
Offset money is your money in a transaction account — spend it tomorrow, no questions. Extra repayments belong to the bank until you redraw them, and redraw is a feature the bank controls, not a right. Lenders can reduce or freeze redraw at their discretion, and several did exactly that to borrowers in 2020. If the money might be needed — job loss, renovation, a business — the offset's unconditional access is worth having.
Difference two: discipline
The offset's flexibility is also its weakness. Money you can spend with a tap tends to get spent; an offset balance that was supposed to grow can plateau for years without anyone deciding anything. Extra repayments add friction — and for many households that friction is the strategy. If you know your own behaviour, weight this honestly: the mathematically identical option is not behaviourally identical.
Difference three: tax, if the home ever becomes a rental
This is the one that costs real money and is nearly impossible to fix afterwards.
Interest is deductible based on what the borrowed money was used for. If you pay down your loan and later redraw to buy your next home (converting the old one to a rental), the redrawn portion was used for a private purpose — its interest is not deductible against the rent, even though it's secured on the rental. You've shrunk the deductible loan permanently.
With an offset, the loan balance never fell. Move the offset cash to your new home instead, the old loan reverts to its full balance, and all of its interest remains deductible against the rental income.
If there is any realistic chance your current home becomes an investment property — a common Australian path — favour the offset and keep the loan intact. This single decision can be worth thousands per year, at your marginal rate, for the life of the loan. (See how the deduction flows through in the negative gearing guide.)
Difference four: cost and availability
Full offset accounts usually come with package fees ($300–$400/year) or slightly higher rates, and are standard on variable loans but rare and restricted on fixed rates. The offset needs a meaningful balance to beat its own fee: at 6%, a $350 annual fee costs you the interest saving on roughly $6,000. If your buffer is small and stable, a fee-free loan with extra repayments may win outright.
The hybrid most people should actually run
- Keep your emergency fund and short-horizon savings in the offset — money you might need keeps full access while earning the loan rate.
- Once the buffer is comfortably sized (three to six months of expenses is the usual anchor), direct surplus to extra repayments if the friction helps you, or keep stacking the offset if the rental-conversion scenario is plausible.
- Recheck at every rate change or refinance. A refinance resets the fine print; the refinance break-even calculator shows whether a lower rate elsewhere beats your current setup even after fees.
The bottom line
Identical interest maths, different everything else. Choose the offset for access and for any home that might become a rental; choose extra repayments for discipline and fee-free simplicity; and remember that either one, funded consistently, beats the perfect choice funded occasionally.
Illustrations use a 6% variable rate for round numbers; your rate changes the size of every figure but none of the logic. General information, not financial advice.