Prepayment Models and Offset Behaviour

Key takeaways

• Prepayment shortens loan life, which affects funding and hedging.
• Offset balances reduce interest without reducing the contractual balance.
• Refinancing is the largest driver of prepayment in Australia.
• Aggregate offset and redraw buffers are a key financial-stability indicator.

Why does prepayment matter to a lender?

Because a 30-year loan almost never lasts 30 years. Borrowers refinance, sell, or pay down faster than the schedule requires, and the average life of a mortgage is far shorter than its contractual term.

That matters for three reasons. Funding has to be matched to expected life, not contractual life. Fixed-rate lending is hedged for an assumed duration, and if borrowers exit early the hedge is wrong. And upfront acquisition costs are recovered over the life of the loan, so a shorter life means less margin to recover them from.

It is also why clawback exists on broker commission: a loan repaid quickly never earns the lender enough to justify what it paid to acquire it.

What drives prepayment?

The seasoning pattern is characteristic: prepayment is low in the first year or two, when switching costs and clawback discourage moving, rises through the middle years as borrowers become rate-aware and equity builds, then falls again as balances become small enough that switching is not worth the effort.

What makes the Australian case unusual?

Offset accounts. In many markets, a borrower who wants to reduce interest must repay principal, which is irreversible. In Australia, money in an offset reduces the interest charged while remaining fully accessible.

That creates a modelling problem: the balance the borrower economically owes and the balance they contractually owe are different numbers. From a funding perspective the money has left; from a contractual perspective the loan is unchanged and the borrower can withdraw it tomorrow.

It also creates a valuable indicator. Aggregate balances in offset and redraw are watched closely as a measure of household resilience, because they represent a buffer households can draw on when income falls or rates rise. The RBA reports on these buffers in its Financial Stability Review.

Two cautions on reading that aggregate. The buffer is unevenly distributed - large balances held by borrowers who were never at risk can mask thin buffers among those who are. And redraw is not equivalent to offset: a lender can reduce or suspend a redraw facility, whereas offset funds sit in the borrower's own account. For a household, that distinction is the difference between a buffer that is definitely there and one that probably is.

Frequently asked questions

How long does the average Australian mortgage actually last?

Far less than its contractual term, because borrowers refinance, sell or repay early. Lenders model expected life rather than contractual term for funding and hedging purposes.

Does money in an offset count as a prepayment?

Economically yes - it reduces the interest charged - but not contractually, since the loan balance is unchanged and the funds remain accessible. That distinction is what makes Australian prepayment modelling unusual.

Why do lenders care about early repayment?

Because loan life determines how funding is matched, how fixed-rate exposures are hedged, and over how long acquisition costs are recovered. A loan repaid quickly may never recover what it cost to write.

Is redraw as good as an offset for emergencies?

Not quite. Redraw is a facility the lender can reduce or suspend, while offset funds are your own money in your own account. For emergency buffer purposes, offset is the more reliable of the two.

Related reading

• Offset Account vs Redraw: The Real Difference
• Offset Account Maths: Dollar-for-Dollar Savings Modelled
• Basel Risk Weights and Your Mortgage

Sources

• Financial Stability Review - household mortgage buffers — Reserve Bank of Australia
• Quarterly authorised deposit-taking institution property exposures — APRA