Mortgage Offset vs Savings Account: The After-Tax Maths

If you hold a variable-rate Australian home loan and keep a cash buffer: an emergency fund, sinking funds for quarterly bills, or savings for a renovation — that money is quietly making a decision for you. Every dollar sitting in a savings account earns taxable interest. Every dollar sitting in a 100% mortgage offset account earns nothing on paper but saves you mortgage interest that is never taxed, because it was never income.

The comparison is not close in most cases. Here is the after-tax maths, the situations where a savings account genuinely wins, and how to model your own numbers.

The Core Insight: Saved Interest Is Untaxed

Interest you earn is assessable income taxed at your marginal rate (plus the 2% Medicare levy). Interest you save is not income at all. It is an expense that never existed.

Scenario	Gross benefit	Tax payable	Effective return
4.5% HISA at 32% marginal rate + Medicare	$1,800 interest	-$612 tax	3.06%
4.5% HISA at 47% top marginal rate	$1,800 interest	-$846 tax	2.39%
Offset against a 5.9% home loan	$2,360 interest saved	$0	5.90%
Offset against a 6.4% home loan	$2,560 interest saved	$0	6.40%

At a 32% marginal rate, your offset needs to beat the after-tax savings rate, which on a 4.5% HISA is only about 3.06%. Since variable home loan rates currently sit well above that, the offset wins on every dollar, every day the cash sits there.

offset effective return

5.90%

Equals your loan rate — guaranteed and untaxed

4.5% HISA after 32% tax

3.06%

$1,800 gross interest − $612 tax on $40k

year-1 offset advantage

$1,136

Extra after-tax value per $40,000 parked

The Compounding Bonus Most Comparisons Miss

The year-one comparison understates the offset benefit. Every dollar of interest not charged flows into principal reduction, which shrinks the interest base for the following month. A persistent offset balance shortens the loan itself.

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On a $650,000 loan at 5.9% with 25 years remaining, a persistent $40,000 offset saves roughly $2,360 in avoided interest in year one alone. Because the loan repays sooner, the lifetime saving runs into the tens of thousands of dollars. Our Mortgage Offset Calculator simulates the full month-by-month amortization for your exact loan so you can see the actual payoff-date difference.

When a High-Interest Savings Account Actually Wins

Offset is not universally better. A HISA (or term deposit) can be the right home for cash when:

• You have no home loan, or a fixed-rate loan. Most fixed-rate Australian mortgages either lack an offset facility entirely or only allow a partial offset (for example, 40% of the balance). A partial offset cuts the effective benefit proportionally.
• Your offset product charges a premium. Some loans add a higher rate or a monthly package fee for offset features. If the fee exceeds roughly $10–$15/month per $10,000 of average offset balance, a free HISA can beat it.
• The cash is not really spare. If there is a real chance you will spend the buffer on a holiday, keeping it psychologically "out of sight" in a separate savings account can be worth the small rate sacrifice.
• You are on a low marginal rate. Below the tax-free threshold and effective LITO zone, the tax cost of HISA interest shrinks toward zero, though your loan rate would still usually beat the gross HISA rate anyway.

Offset vs Redraw vs Paying Down the Loan

Putting the same $40,000 inside the loan as extra repayments saves the same interest as offsetting it, but the money stops being yours to access freely. Redraw usually requires a request, can carry minimums, and is not guaranteed.

For investors, there is a second-order consideration: parking personal cash in an offset against an investment-property loan preserves deductible interest if you later convert the property, whereas paying it down permanently reduces future deductible debt. That is a structural decision worth discussing with an accountant.

Where to Put the Buffer: A Practical Order

Run Your Own Numbers

The Mortgage Offset Calculator takes your loan balance, rate, remaining term, and the cash you could park, then simulates the full amortization schedule both ways. You get the exact interest saved, the months or years cut from the loan, and the after-tax comparison against a savings account at your marginal rate.

Once you have the answer, the natural next step is turning the buffer into a tracked goal: earmarking it as an emergency fund or a bill sinking-fund balance inside PFinance Goals so a "spare" $40,000 does not dissolve into general spending.

If you are still deciding whether to prioritise the mortgage at all (versus attacking a HELP debt or investing), our HECS-HELP repayment guide and emergency fund guide cover the trade-offs.