Once you have a mortgage, two levers make the biggest difference to what it ultimately costs: paying a little more than you have to, and offsetting savings against the balance.
Interest is charged on the balance, so early payments matter most
In the early years of a loan, most of each repayment is interest and only a small slice reduces principal. Any extra dollar you pay goes entirely to principal — and because it reduces the balance every subsequent period is calculated on, its effect compounds for the remaining life of the loan.
This is why an extra $100 a fortnight in year one saves far more than the same $100 in year twenty. More frequent repayments reduce the principal sooner, so less interest accrues.
How an offset account works
An offset account is a transaction or savings account linked to your mortgage. Its balance is subtracted from your loan before interest is calculated. With a $500,000 loan and $50,000 offset, you're charged interest as though you owed $450,000.
The advantage over simply paying down the loan is access: the money stays yours and can be withdrawn any time. That makes an offset a sensible home for an emergency fund — it earns you the equivalent of your mortgage rate, tax-free, while staying liquid. A revolving credit facility works on a similar principle.
Lump sums
A one-off lump sum — an inheritance, a bonus, a work payout — behaves like a permanent step down in your balance. Applied early, a modest lump sum can remove years from the term. Check your loan's conditions first: fixed-rate loans often limit extra repayments and may charge a break cost.
Shortening the term versus reducing the repayment
When your circumstances improve, you can either keep the repayment the same and finish earlier, or reduce the repayment and keep the term. Keeping the repayment steady is what produces the large interest savings — and one straightforward version of this is switching from monthly to fortnightly payments, which adds roughly an extra month's worth of repayments each year.
What fixed rates allow
Most of this assumes you can overpay freely, and on a floating loan you generally can. Fixed loans are different: lenders commonly allow extra repayments only up to a capped amount each year, with a break cost beyond it. The cap is often generous enough for a regular fortnightly top-up and too small for a large windfall. Before you plan a strategy around overpayments, find out what your specific loan permits — and if you expect a lump sum, that is an argument for leaving a slice of the mortgage floating or on offset rather than fixing the lot.
Where a spare dollar should go first
Extra mortgage payments are not automatically the best use of surplus cash. A dollar aimed at the mortgage saves you the mortgage rate; a dollar aimed at a credit card or personal loan saves you a much higher one, so expensive debt comes first. After that, an emergency buffer usually beats overpaying, because a household with no cash reserve ends up borrowing at card rates the first time something breaks — undoing the saving and then some. Once those two are handled, overpaying the mortgage is a reliably good, low-risk return. The order matters more than the amount.
Test yourself against a higher rate
Before committing, look at what your repayment would be at a rate two or three points higher. Banks do this as a matter of course, and it is a useful private sanity check on whether a loan is comfortable or merely possible.