Offset account vs savings account: the after-tax break-even test

Data checked: 2026-09-02

An offset account and a high-interest savings account can hold the same dollars but produce different cash-flow results. The offset reduces the mortgage balance on which interest is charged. The savings account earns interest, which is generally assessable income and therefore part of the tax calculation. The useful comparison is therefore not mortgage rate versus savings rate. It is net mortgage interest saved versus savings interest kept after tax, after allowing for fees and any mortgage-rate premium.

This article is general information, not personal financial, credit, legal or tax advice. It provides a worksheet, not a product recommendation.

Start with the two different cash-flow mechanisms

ASIC’s Moneysmart describes an offset as a transaction account linked to a home loan. The lender subtracts the offset balance before calculating mortgage interest. A high-interest savings account instead pays interest on the deposit.

That distinction matters at tax time. The Australian Taxation Office says bank interest is assessable income. Its ruling on conventional loan-offset arrangements explains that the deposit side generally does not earn interest; the benefit is lower loan interest instead.

For a private home loan, one dollar in a fully functioning offset therefore earns an implicit return close to the mortgage rate before product costs. One dollar in a savings account earns the advertised deposit rate, then loses the portion payable as tax. Neither statement is a guarantee because balances, rates and product conditions change.

Use an after-tax formula, not two headline rates

The first-pass annual comparison is:

Offset benefit = average eligible offset balance × mortgage rate

Savings benefit = average savings balance × savings rate × (1 − marginal tax rate)

Net offset advantage = offset benefit − savings benefit − annual offset fees − mortgage-rate premium cost

The mortgage-rate premium cost is the extra rate charged on the whole loan for choosing an offset-eligible package. If the offset loan is 0.10 percentage points dearer on a $500,000 balance, that cost is about $500 a year before the loan balance changes. It must not be ignored merely because it is not labelled an account fee.

A worked example shows why the fee matters

Assume a private home loan rate of 6.00%, a savings rate of 5.00%, a 32% marginal tax rate—the tax rate applied to the next dollar of income—for the illustration, an annual package fee of $395 and a 0.10 percentage-point mortgage-rate premium on a $500,000 loan.

The savings account keeps 3.40% after the assumed tax rate. The offset’s rate advantage before product costs is 2.60 percentage points. The annual fixed and rate-premium costs total about $895. Dividing $895 by 2.60% gives a first-pass break-even average balance of about $34,400.

These figures are illustrative, not current product quotes. Replace every input with the rate, tax position, loan balance, fees and average balance that actually apply to you.

Average daily balance is more useful than the peak balance

Moneysmart notes that mortgage interest is generally calculated daily. A $50,000 balance that sits in the offset for two days does not produce the same annual benefit as a stable $50,000 balance.

Use transaction history to estimate the average amount left after bills, card repayments and large withdrawals. If salary enters the account and most of it leaves within days, a month-end screenshot can overstate the benefit. A three- or six-month average provides a more defensible first input.

The same discipline applies to a bonus savings account. If the advertised rate depends on monthly deposits, limited withdrawals or balance growth, use the rate you are likely to keep, not merely the maximum advertised rate.

Fees and mortgage-rate premiums can reverse the result

Moneysmart specifically warns that offset features may carry higher interest rates, annual package fees or monthly account fees. Compare the offset loan with a genuinely similar loan without the feature, rather than assuming the mortgage rate would be identical.

List each cost separately:

• annual package or account fee

• monthly offset fee

• mortgage-rate difference on the full loan balance

• any lost discount or cash benefit

• switching or refinancing cost if a new loan is required

The existing mortgage offset after refinancing audit explains how to check that the account remained linked after a loan change. The break-even calculation here assumes that linkage has already been verified.

An offset must be linked and eligible to count

ASIC found that customers can pay more interest without noticing when an offset is not linked correctly. Repayments may stay unchanged even though the expected interest reduction is missing. Refinancing or switching products can also break the link.

Check the bank app, statement and loan documentation for the linked account, eligible loan portion and offset percentage. A partial offset should use only the eligible percentage in the formula. An offset attached to one split does not automatically reduce interest on every split.

If the statement does not show how interest was reduced, ask the lender for a written explanation. A planned benefit is not evidence that the feature is operating.

Liquidity and behaviour still matter

Both accounts can offer access to cash, but the behavioural result can differ. An offset integrated with everyday spending may make money easy to use. A separate savings account may create a clearer boundary for an emergency fund. The mathematically stronger option can still fail if it causes the average balance to fall.

The three buffers first-home buyers should not confuse article separates transaction cash, an emergency reserve and a repayment buffer. That distinction remains useful here: not every dollar should be judged only by its interest rate.

Investment debt needs a separate tax boundary

This worksheet is designed for cash held against a private home loan. Do not automatically transfer the conclusion to an investment loan or mixed-purpose debt. Interest deductibility depends on the use of borrowed funds and the account structure, not simply the property offered as security.

An offset and redraw are also different. Moneysmart describes an offset as a separate transaction account, while redraw is access to extra repayments made into the loan and remains subject to loan terms. Moving money through mixed-purpose debt can have tax consequences that this general comparison does not resolve. Seek registered tax advice for that boundary.

Build your own break-even worksheet

• Record the current loan balance and the rate on the eligible split.

• Obtain the best comparable loan rate without an offset.

• Add annual, monthly and switching costs.

• Estimate the average cash balance from actual transactions.

• Record the savings rate you can realistically retain.

• Apply the tax rate relevant to your assessable savings interest.

• Calculate the break-even average balance.

• Confirm the offset is linked and operating.

• Rerun the calculation after a rate, fee, loan or tax change.

For the simple case, the break-even balance is:

Annual offset costs ÷ [mortgage rate − savings rate × (1 − tax rate)]

If the rate gap inside the brackets is zero or negative, the offset does not recover its extra cost under those inputs. If the gap is positive, the formula shows the average balance required before the offset begins to lead.

Know what would change the answer

The savings account becomes more competitive if its retained after-tax rate rises, the offset loan carries a larger rate premium, fees increase or the average cash balance falls. The offset becomes more competitive if the mortgage rate rises relative to the retained savings rate, product costs fall or a stable balance grows.

The result should also be rejected if the offset is not linked, the relevant loan split is ineligible, the product offers only a partial offset, or a bonus savings condition is unlikely to be met. These are contract and behaviour checks, not forecasting questions.

Conclusion

An offset account does not automatically beat a high-interest savings account, and the highest advertised savings rate does not automatically win either. Compare mortgage interest saved with savings interest retained after tax, then subtract the offset’s fees and any mortgage-rate premium.

The result depends most heavily on a stable average balance, the whole-of-loan cost of the feature and whether the account is actually linked. Treat the break-even number as a review rule and update it whenever rates, fees, loan structure or tax circumstances change.

Sources checked

• ASIC Moneysmart — Mortgage offset accounts

• ASIC Moneysmart — Pay off your mortgage faster

• ASIC Moneysmart — Save for a house deposit

• Australian Taxation Office — TR 93/6 loan account offset arrangements

• Australian Taxation Office — Interest income

• ASIC — Mortgage offset account practices

The author or publisher may hold a direct or indirect interest in a financial product or institution mentioned in this article. This article contains no sponsorship, affiliate arrangement or paid recommendation; any display advertising is separate from editorial judgement. All investing and borrowing decisions can produce loss, and product, tax and credit suitability should be checked with the relevant lender and a properly qualified adviser.