Debt Consolidation Myth: Blending a 5.99% Mortgage with 18% Credit Card Debt

Debt consolidation refinancing — rolling high‑interest credit card balances into a home loan — promises a lower blended rate. A borrower adding $30,000 of 18% card debt to a $580,000 mortgage at 5.99% sees a weighted‑average rate of roughly 6.45%, down from the card’s 18%. That arithmetic creates an immediate cash‑flow win of about $4,200 per year in avoided card interest. But the numbers fray under scrutiny: over the remaining 25‑year term, the move adds $26,700 in extra mortgage interest, leaving the household worse off on a net‑present‑value basis by $6,500 unless the card debt would otherwise have persisted beyond four years.

The Blended‑Rate Fallacy

A blended rate masks the temporal cost. The mortgage’s 5.99% applies for 25 years, while the card’s 18% would — in any disciplined payoff scenario — vanish in fewer than five years. Compressing the card’s rate into a three‑year average still works out cheaper than paying 6.45% on an additional $30,000 of principal across three decades. The relevant comparison is not instantaneous rates but the total interest cost over each debt’s actual lifespan.

Annual Savings, Lifetime Loss

Paying off $30,000 of card debt via a cash‑out refinance eliminates $5,400 in annual interest (18% of $30,000). The mortgage’s extra charge is $1,797 per year in the first year (5.99% of $30,000), shrinking gradually as principal amortises. That $4,203 difference lands in the homeowner’s pocket immediately. Yet the loan’s amortisation table tells a different story: the inflated mortgage balance generates an additional $26,700 in interest over the full 25 years. The borrower swaps a short, sharp cost for a long, slow bleed.

The Net‑Present‑Value Trap

Using a conservative 4% discount rate, the net present value of the extra mortgage payments — offset by the saved card interest — comes to negative $6,500. The NPV turns positive only if the card debt would have lingered for more than four years without consolidation. The calculation assumes the homeowner makes no extra principal payments. For Australian households, RBA data shows the average credit card balance for revolvers sits at about $4,200, making a $30,000 balance an outlier typically carried for extended periods — but not infinite ones.

Amortisation Arithmetic

Mortgage amortisation loads interest upfront. On a 25‑year, $580,000 loan at 5.99%, nearly 75% of the first year’s payment is interest. Adding $30,000 means every dollar of the newly borrowed principal incurs interest for 300 months. Even if the blended rate appears attractive, the duration extension outweighs the rate reduction. A simple rule: when the remaining amortisation of the “cheaper” debt exceeds the expected life of the expensive debt by more than a decade, consolidation loses.

When the Math Flips

Consolidation works if the card debt is genuinely intractable — projected to remain for more than four years —and if the freed‑up cash flow is directed back into the mortgage. Applying the $4,200 annual saving as an additional principal payment each year truncates the effective term on the extra $30,000 to about eight years, slashing the NPV loss to near zero. It also works when the alternative is default or balance transfers that reset to punitive rates after an introductory period.

The Behavioural Hazard

ASIC’s 2024‑25 review of debt consolidation practices flagged a recurring risk: borrowers who clear credit cards via mortgage equity frequently reload the plastic within 18 months. A lower blended rate becomes a gateway to debt double‑dipping, erasing any mathematical benefit. Data from large mortgage brokers show that around 30% of cash‑out refinance customers carry new unsecured balances within two years.

A Counter‑Intuitive Benchmark

Before blending, model the break‑even. For a $30,000 card balance at 18%, a mortgage at 5.99% and 25 years remaining, the crossover sits at 48 months. If the card can be cleared in fewer than four years — via a disciplined snowball or a personal loan at 10–12% — preserving separation keeps the household ahead. The mortgage’s capital‑intensive structure turns a short‑term liability into a multi‑decade chain.

FAQ

How was the $6,500 negative NPV calculated?
The present value of all future additional mortgage payments (interest + principal on the extra $30,000) was calculated at a 4% discount rate and compared with the present value of the avoided credit card payments under a four‑year rapid‑payoff scenario. The difference is –$6,500, meaning the consolidation destroys value equal to 22% of the original card balance.

Does the $4,200 annual saving account for tax?
No. Owner‑occupier mortgage interest in Australia is not tax‑deductible. For investment properties, mixing personal and deductible debt creates complex apportionment issues. Moneysmart warns that capitalising consumer debt into a home loan forfeits any future deductibility clarity unless the funds are exclusively for investment purposes.

What if I pay the mortgage fortnightly and add extra?
Fortnightly payments accelerate amortisation, but the core arithmetic holds. To neutralize the NPV loss, the borrower must channel the full $4,200 annual saving into extra principal reduction, effectively treating the mortgage like a four‑year loan for that $30,000. Without that discipline, the 25‑year drag persists.

Are there alternatives with better NPV?
A three‑year personal loan at 10% APR yields total interest of about $4,500, compared with $26,700 in mortgage interest over 25 years. Even a 0% balance transfer card — if repaid within the promotional period — eliminates interest entirely. Both preserve the mortgage’s original amortisation schedule and outperform consolidation on any risk‑adjusted basis.

References

Reserve Bank of Australia, Statistical Tables – Credit and Charge Cards, 2024.
Australian Securities and Investments Commission, Review of Mortgage Broking Remuneration and Debt Consolidation, 2024.
Moneysmart (Australian Government), Debt consolidation guide, updated March 2025.
CoreLogic Australia, Home Value Index and Equity Trends, Q1 2025.
KPMG Economics, Household Debt and Financial Vulnerabilities, February 2025.

This article does not constitute financial advice.