Quick answer: There is no universally best split. Match the fixed portion to the repayments you want certainty on and keep enough variable flexibility for offset, extra repayments or planned changes.
Fixed, variable or split: what are you really choosing?
The decision is a trade-off between certainty over some repayments, the ability to benefit if market rates fall and flexibility to make extra payments, refinance or use an offset account. A fixed-rate loan locks its contracted rate for an agreed period, often one to five years. A variable rate can change during that period. A split divides the mortgage into separately priced portions.
A split is not inherently cheaper than a fully variable loan. It lets a borrower choose how much debt is exposed to rate changes, but the fixed and variable portions can carry different fees, features, repayment rules and future reset dates.
ASIC's Fixed vs Variable Home Loan Rates, updated September 2026, documents fixed-period repayment certainty, potential break fees, offset limits, variable rate risk and the mechanics of split mortgages.
Worked example: a $600,000 loan split 50/50
Assume $300,000 fixed at an illustrative 5.70% and $300,000 variable at 6.20%. For a rough first-year interest sensitivity holding principal constant:
| Scenario | Fixed portion interest | Variable portion interest | Total indicative annual interest |
|---|---|---|---|
| Starting split | $17,100 | $18,600 | $35,700 |
| Variable rate rises 1 percentage point | $17,100 | $21,600 | $38,700 |
| Variable rate falls 1 percentage point | $17,100 | $15,600 | $32,700 |
The 1-point rate movement changes the annual interest exposure by approximately $3,000 on the variable half, compared with $6,000 if the entire $600,000 loan were exposed to the same rate movement. The fixed portion remains at 5.70% during its fixed period.
These are interest-only sensitivities, not principal-and-interest repayment quotes: actual balances decline with repayments, interest is typically calculated daily, and the fixed period eventually ends. Use the Mortgage Repayment Calculator to model repayment schedules at differing rates.
How much should you fix?
Instead of choosing 50/50 because it sounds balanced, consider three amounts:
- The debt whose repayment you want stable for the next one to five years.
- The amount you can keep variable to accommodate savings, offset, extra payments or an anticipated refinance.
- The likely borrowing balance remaining when the fixed rate ends. This is where the lender's revert rate matters.
A borrower expecting an imminent sale or a large lump-sum repayment may value variable flexibility more than someone prioritising short-term budget certainty. Conversely, someone with little cash-flow headroom may prefer shielding an agreed portion from near-term rate increases while accepting that fixing prevents benefits from falling rates on that portion.
A split with offset: can the variable portion absorb your savings?
Suppose you hold $120,000 in an eligible offset account, but your variable split is only $100,000. Assuming the offset is linked solely to that variable split and is a standard full offset, only up to $100,000 can offset interest on that particular loan balance. The extra $20,000 may sit as deposit cash without reducing interest on the fixed portion.
If the variable split were $300,000, the same $120,000 could reduce the interest-bearing balance on that portion to roughly $180,000, subject to its actual eligibility and daily balances.
Before fixing most of a mortgage, verify exactly which split can be linked to offset, whether any fixed portion has offset access, and whether an annual package or rate premium changes the net advantage. See Offset vs Redraw and the Offset vs Extra Repayment Calculator.
What happens when the fixed period expires?
When a fixed period finishes, the facility generally moves to the lender's variable revert rate unless another arrangement is made. That rate may be materially different from the original fixed rate. Ask for a clear projected repayment based on the remaining balance and remaining term, not the full original term.
Example: A borrower fixes for three years on a 30-year loan. At expiry there are approximately 27 years left, not a fresh 30-year repayment period. When comparing new pricing, use the actual balance and remaining term. An extension back to 30 years could lower required repayments while increasing total interest and delaying principal reduction.
What if you sell or refinance before the fixed term ends?
Early exit, repayment beyond an allowed threshold or switching products during the fixed period may trigger a break cost. It is not simply a fixed percentage of the original loan: calculation and entitlement depend on the lender's contract and market-rate circumstances.
Get a written payout or break-cost figure for the proposed date. Compare the alternative lender's full saving against that cost using the Refinance Break-Even Calculator. Moneysmart's Switching Home Loans also highlights break fees and term changes.
Where green-home finance fits in
A green or sustainable-upgrades home loan is generally a product or purpose-based funding choice, not a fourth interest-rate type alongside fixed and variable. It may finance eligible solar, batteries, insulation, electric hot-water or other efficiency upgrades under lender-specific conditions.
For example, Westpac's Sustainable Upgrades Home Loan currently describes a separate property-secured loan of $4,000–$50,000, up to 10 years, with principal-and-interest repayments and no offset/package on that facility. It is supported by the Clean Energy Finance Corporation's Household Energy Upgrades Fund. These are Westpac-specific published features, subject to current availability and eligibility—not universal green-loan requirements or a guaranteed offer.
Worked example: $20,000 in home energy upgrades
Assume $20,000 for a solar and battery project. A ten-year loan at an illustrative 5% nominal interest has a scheduled repayment around $212/month, with total interest around $5,456 over 120 months. Using a 25-year mortgage schedule at the same 5% would bring required repayments down to approximately $117/month, but total interest would be approximately $15,075 if left for the full term.
Both are merely financing scenarios. An actual energy upgrade decision also requires quotes, rebates, expected consumption savings, warranties and replacement costs. A lower annual rate does not make a 25-year repayment term suitable for equipment with a materially shorter economic life.
Before comparing a green-home facility with a normal mortgage top-up, check eligible improvements, minimum/maximum facility, security, repayment term, early payout provisions, fees and whether an offset is supported. Read Mortgage Top-Up vs Separate Finance for the term-versus-total-cost comparison.
Borrower questions answered
If interest rates are expected to fall, should I stay variable?
A falling-rate view alone is not a complete borrowing decision. Consider the rate already priced into a fixed offer, the ability to handle increases, your likely holding period and restrictions on changing the loan. Rate forecasts can be wrong.
Is a 70/30 split better than 50/50?
Not universally. A larger fixed share limits rate-change exposure on more debt but may restrict offset and early repayments. Choose proportions to match realistic cash reserves and how much repayment certainty you require.
Can I fully offset a split loan?
Potentially the eligible linked portion, not necessarily all splits. Check the lender's exact offset linkage, fixed-rate restrictions and how the account balance is applied.
Can I make unlimited extra repayments on a fixed loan?
Do not assume so. Many fixed products impose thresholds or restrictions and may charge costs for certain early repayments. Confirm the contract.
Can I refinance the variable part but keep the fixed part?
Changing just one split may be possible in some structures, but security arrangements, contracts and lender acceptance matter. Do not assume a split automatically makes independent lender refinancing straightforward.
Is a green home loan cheaper than an ordinary mortgage?
It can be attractive where a published eligible upgrade facility offers favourable pricing, but compare total interest over the intended repayment term plus fees. Eligibility and benefits depend on the current product, not the word 'green'.
Does interest-only improve the split decision?
Interest-only affects repayment structure, not whether a rate is fixed or variable. It can reduce required payments for a period while leaving the principal outstanding. The subsequent principal-and-interest payment may be significantly higher when the IO period ends.
Practical comparison checklist
Compare fixed and variable rates, fixed-term expiry, projected revert rates, annual and per-split fees, allowed extra repayments, genuine offset benefits, potential break costs and the remaining debt at your expected refinance or sale date. For green funding, also compare eligibility, purpose restrictions and the useful life of the equipment.
Explore the Home Loans Knowledge Centre for the five core decision calculators, or Discuss a Fixed, Variable or Split Loan Structure.
Match the variable split to the cash you expect to retain
If only the variable split has a linked offset, cash above that split balance may provide no additional mortgage interest reduction. Model a realistic savings path before fixing most of the debt, then check the actual product terms for partial offsets, extra repayments and early payout.
Use the repayment calculator for the variable portion, and read offset versus extra repayments for fees and access trade-offs. A whole-loan rate assumption should not be presented as a forecast for two differently priced splits.
Sources and verification
- Moneysmart — Switching home loans
- Moneysmart — Mortgage offset accounts
- APRA — Macroprudential policy settings
These sources support the general mechanics and decision framework. Product availability, pricing, fees and lender policy can change. Tax-sensitive decisions should be checked against current ATO guidance and, where appropriate, a qualified tax adviser or accountant.
Related GPS Finance resources
- Is Refinancing Your Home Loan Worth It? Break-Even, Fees and Loan Term
- Offset vs Redraw: Which Is Better for Flexibility, Tax and Future Plans?
- Mortgage Top-Up vs Separate Finance for Cars, Renovations and Other Major Purchases
- Mortgage options
- Offset vs Extra Repayment Calculator
About the author: Krish Neelamraju is a finance and credit professional and founder of GPS Finance Group.
General information only. It is not personal financial, tax or legal advice. Finance approval, pricing, terms and structure are subject to lender assessment and the borrower’s circumstances.
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