Interest saved by offsetting savings against the loan.
Interest saved per year
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Balance interest is charged on
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Your breakdown
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Offset and revolving credit, and how they differ
An offset facility links your everyday savings to your mortgage so the bank charges interest only on the net balance, the loan minus whatever you have parked alongside it. Hold $40,000 of savings against a $500,000 loan and you pay interest on $460,000. The cash stays yours, fully accessible, and it quietly cuts your interest bill while it sits there. A revolving credit facility works on the same principle but as a single large overdraft: your salary lands in it, your spending comes out of it, and the balance you owe drops every time your pay clears.
This tool isolates the simplest, most reliable measure of the benefit, the interest you avoid in a year. It does not project a shortened term, because how fast the loan falls depends entirely on how much you leave in the account month to month, which only you can predict.
The return on parking $40,000 against a 6.5 percent loan
The maths is refreshingly direct. Multiply the offset balance by your mortgage rate and you have the interest saved over a year. The clever part is what that figure represents. Saving 6.5 percent of interest is economically the same as earning a 6.5 percent return on that $40,000, except this return is completely tax-free.
Now compare that $2,600 to leaving the same $40,000 in a term deposit. At a 5 percent deposit rate you might earn $2,000 of interest, but that interest is taxable. At a 30 percent marginal rate you keep about $1,400 of it. The offset delivers $2,600 with no tax to pay, so the gap in your favour is real money.
When an offset beats every other home for your cash
The decision rule is simple. If your mortgage rate is higher than the after-tax return you could earn on savings elsewhere, and for most households it is, then your emergency fund, your house deposit-in-waiting and any spare cash do more good sitting in an offset than in a savings account. New Zealand has no general capital gains tax, but interest income is fully taxable, which is exactly why a tax-free interest saving is so attractive next to a taxable one.
Two cautions from experience. First, offset and revolving facilities sometimes carry a slightly higher interest rate or a monthly fee than a plain fixed loan, so the saving has to clear that hurdle. Second, the discipline matters more than the structure. A revolving credit account only works if you genuinely live below your income and let the balance fall. If the account creeps up each month because spending fills the space, you have simply built yourself an expensive overdraft. The people who win with offset are those who keep a real, stable buffer parked against the loan.
Does the interest saved count as taxable income?
No. You are not earning interest, you are avoiding it, so there is nothing for IRD to tax. That is the structural advantage over a savings account, where resident withholding tax is deducted from the interest you earn before it even reaches you.
Can I offset against only part of my mortgage?
Yes, and many people do. A common setup is to keep most of the loan on a low fixed rate and carve out a smaller revolving or offset portion for the savings to work against. You get the certainty of the fixed rate on the bulk of the debt while still earning the tax-free saving on your buffer. Your bank can split the loan this way when you set it up or refix. As a rule of thumb, size the offset portion to roughly the amount of cash you reliably keep on hand, your emergency fund plus a typical month of salary, so the savings are always working against the loan rather than sitting idle in a separate account earning taxable interest.