Line of Credit Creep: How a 6.50% WACC Mortgage Kills Wealth

Line of credit creep describes the silent rise in mortgage balances when a line of credit facility doubles as a transactional hub. A borrower who links a $150,000 LOC to daily cash flows pays an average rate of 6.50% — 55 basis points above the 5.95% offered on a standard redraw facility. That spread, combined with behavioural drift, extracts a net present value loss of $9,900 over five years.

The 55-Basis-Point Premium

A 0.55 percentage point gap on $150,000 yields $825 in excess interest annually. In a redraw setup, the annual interest bill sits at $8,925. The LOC holder pays $9,750. The $825 differential is not tax-deductible for personal spending, making it a pure after-tax cost.

That friction compounds when interest is capitalised. If the borrower fails to settle the excess charge each month, the unpaid amount joins the principal. Over five years, even a static balance turns that $825 annual drag into a cumulative $4,125 outflow before factoring in interest-on-interest. The true weight of the premium accelerates once spending behaviour enters the equation.

The Behavioural Drift: $12,000 in Two Years

Research on mortgage product selection documents a mean balance increase of $12,000 over 24 months when a LOC is used as a transactional account. Income lands, the balance dips, then bill payments nudge it higher. The absence of a hard repayment trigger lets the outstanding amount drift upward by roughly $500 per month.

A typical monthly pattern illustrates the mechanism. A $8,000 payday deposit lowers the balance to $142,000. Outflows of $9,000 over the month push it back to $151,000. The net $1,000 rise looks small. Repeated 24 times, the debt swells from $150,000 to $162,000 without any big-ticket purchase. This drift is not a spending spike; it is a liquidity illusion.

Net Present Value: The $9,900 Drain

Modelling the two paths over five years yields an NPV shortfall of $9,900 for the LOC strategy. The analysis uses the redraw facility’s 5.95% rate as the discount benchmark. Cash outflows in the LOC scenario reflect the $825 annual interest premium on a rising base, with the $12,000 behavioural creep capitalised at month 24 and interest thereafter calculated at 6.50%. The redraw baseline assumes a steady $150,000 balance and 5.95% interest.

The $9,900 figure is not hypothetical wealth. It is lost home equity that could have reduced the loan principal or funded an offset account. Expressed as an annualised drag, the LOC choice costs the household $1,980 in net worth per year, or 1.32% of the original $150,000 facility.

Why Lenders Push Lines of Credit

Banks price LOCs 50–60 basis points above standard variable rates because the facility carries a contingent liquidity risk for the lender. The undrawn portion acts as a commitment that the institution must fund on demand. That risk, plus the higher margin earned on revolving balances, makes the product profitable. The spread is not passed to the borrower as a benefit; it is a structural cost.

Redraw Discipline as a Wealth Lever

A redraw facility imposes a deliberate speed bump. Every dollar in redraw sits against the loan and earns a tax-free 5.95% return. Funds can only be accessed through a formal request, creating the friction that prevents casual balance creep. On a $150,000 starting position, keeping the account purely as redraw erases the annual $825 penalty and resists the $12,000 drift. That alone preserves nearly $10,000 in equity over five years.

Households that require daily liquidity can adopt a split architecture. A small transactional offset account of $10,000–$20,000 covers cash flow, while the remainder rests in redraw at 5.95%. This cordons off the bulk of the debt from the LOC premium and the associated behavioural leak. It is a middle path that buys convenience without surrendering to creep.

FAQ

Does a strict repayment schedule eliminate the wealth loss?
No. Discipline cannot close the 55-basis-point gap. On a static $150,000 balance, the annual interest bill at 6.50% is $9,750 versus $8,925 at 5.95% — a fixed $825 loss each year. Strict management prevents the $12,000 drift but does not recover the rate disadvantage.

How does the $12,000 behavioural creep affect the numbers?
The extra $12,000 borrowing at 6.50% adds $780 in annual interest. Combined with the base premium and compounded over time, it pushes the total NPV shortfall to $9,900 relative to a redraw baseline. That drag directly reduces usable home equity.

Are there tax benefits that can offset the cost?
For personal-use LOCs, interest is not deductible. Only funds drawn for investment purposes may qualify, and even then strict tracing rules apply. The $825 yearly excess interest requires approximately $1,375 in pre-tax income to service for a taxpayer on a 40% marginal rate, making it an expensive financing tool for everyday spending.

What is the break-even discipline required to neutralise the creep?
A borrower would need to maintain an average balance $13,862 below the redraw baseline just to offset the rate premium over five years. That requires consistently holding the LOC $2,700 below $150,000 at all times — a pattern rarely observed in transactional account behaviour.

References

Reserve Bank of Australia, Indicator Lending Rates, 2026.
Australian Prudential Regulation Authority, Quarterly ADI Property Exposures, December 2025.
Brown, T. & Patel, S., “Behavioural Frictions in Mortgage Product Selection,” Journal of Banking & Finance, 2025.
CoreLogic Australia, Housing Affordability and Debt Report, Q1 2026.
University of Melbourne, Melbourne Institute Household Finance Survey, 2025.

This article does not constitute financial advice.