What your repayments could be.

A $600,000 loan at 6% over 30 years has a repayment around $3,600 a month and a total cost of roughly $1.29 million. The interest is more than the house.

On that same loan, an additional $200 a month cuts about five years off the term and saves roughly $150,000 in interest. It's the highest-return thing most borrowers can do with spare money, and it costs nothing to set up.

Interest is charged on the outstanding balance, so a dollar paid in year two does far more than a dollar paid in year twenty. If you're going to make extra repayments, the timing matters more than the amount.

Switching from monthly to fortnightly repayments gets talked about as a clever hack. It mostly works for a simple arithmetic reason.

There are twelve months in a year but twenty-six fortnights. If you halve your monthly repayment and pay it every fortnight, you end up making the equivalent of one extra monthly repayment a year without really noticing. Over a thirty-year loan that takes a meaningful chunk off both the term and the total interest.

Almost all of the benefit comes from paying more each year, not from paying more often. Fortnightly is just a painless way to do it. If you'd rather stay monthly and add a bit extra each time, you should land in much the same place.

Application and ongoing fees, LMI if your deposit is under 20%, offset account balances, and any rate change over thirty years. Rates have moved more than three percentage points in the last five years alone, so a fixed figure over three decades is a useful comparison and a poor prediction.

Don't calculate the repayment you can afford today. Calculate the repayment at two percentage points higher and see whether that still works. That's roughly what the lender does when they assess you, and it's a better test of whether the loan suits your life.