In 2026, ASIC reviewed how eight major Australian banks manage mortgage offset accounts and found that some customers were quietly being charged more interest than they should have been, simply because their offset account wasn't set up or linked correctly. Banks had already paid out more than $55 million in compensation, and more was expected. The unsettling part is that repayments don't change when this happens, so there's often no obvious sign anything is wrong. You just pay more interest, month after month, without knowing.
This calculator exists to make it easy to check the maths yourself. Enter a loan balance, an offset balance, and an interest rate, and it works out what the interest charge should be, showing every step of the calculation so nothing is a black box. If that number doesn't match what's on a bank statement, that's a clear, specific thing to raise with the bank.
It's a simple, free tool. It isn't run by a bank, a broker, or any financial institution, and it isn't financial advice. It was put together because checking this kind of thing shouldn't require specialist knowledge or trust in a black box, just a calculator that shows its working out.