When To Switch Between Interest‑Only and Principal‑and‑Interest

Restructuring from interest‑only (IO) to principal‑and‑interest (P&I) – or the other way around – is really about one big trade‑off: immediate cashflow versus long‑term debt reduction and safety.

If you switch from IO to P&I, your repayments usually jump 30–70%, but your loan starts shrinking and you reduce long‑term interest costs. If you move to or extend IO, you free up cash now but pay more total interest and accept more refinancing risk. In today’s higher‑rate, tighter‑lending environment, you need to assume you may not always be able to extend IO or refinance on demand.

This guide walks through when it makes sense to change, how to run the numbers, and the concrete steps to take this week to choose a structure you can live with even if rates rise another 2–3%.

Interest‑only and principal‑and‑interest loans balance short‑term cashflow against long‑term debt reduction.

1. IO vs P&I in plain English

1.1 What’s the real difference?

Interest‑only (IO)

• You pay just the interest for a set period (often 1–5 years).
• Your loan balance doesn’t fall unless you make extra repayments.
• After the IO period, repayments revert to higher P&I over the remaining term.

Principal‑and‑interest (P&I)

• Every repayment includes interest plus a slice of principal.
• Your balance falls gradually from day one.
• Total interest over the life of the loan is much lower.

Most Australian lenders price P&I cheaper than IO for owner‑occupier loans and, increasingly, for investment loans as well.

1.2 Typical repayment jump when IO ends

Let’s say:

• Loan: $800,000
• Rate (P&I or IO): 6.2% p.a. variable (illustrative only)
• Original term: 30 years

1. During 5‑year IO period

   • Repayments: interest only
   • Monthly: ~$4,133

2. After IO ends (25 years remaining, now P&I)

   • Monthly P&I: ~$5,267

That’s a jump of ~$1,134 per month (+27%). If the rate had also risen during the IO period (as many borrowers have seen since 2022), the jump can easily exceed 40–60%.

For larger or shorter‑term loans, or those already on high rates, we regularly see increases closer to 60–70%, as covered in more detail in our investment‑focused guide: /insights/using-interest-only-periods-strategically-without-forever-mortgage.

1.3 What are you really choosing between?

You’re choosing:

1. Cashflow now vs. wealth later

   • IO: more cash to invest, run a business, or absorb higher living costs.
   • P&I: less spare cash now, more equity and less risk later.

2. Flexibility vs. certainty

   • IO relies on your future ability to refinance or sell on your terms.
   • P&I is safer if banks tighten or property values fall.

3. Behavioural guardrails

   • P&I forces you to build equity.
   • IO only works if you actually save or invest the freed‑up cash – not if it quietly disappears into lifestyle.

2. When it makes sense to move from IO to P&I

2.1 Your IO period is ending and the jump looks scary

If your IO period is about to finish, don’t wait for the lender’s letter.

1. Ask your lender or broker for your revert‑to P&I repayment.
2. Model it at current rate +3% to reflect APRA’s typical buffer.
3. Check what percentage of your after‑tax income that number is.

As a rough guide, for most households:

• Under ~30% of after‑tax income: usually comfortable.
• 30–40%: workable with a clear budget and buffers.
• Over 40–45%: red flag, especially if you’re self‑employed or highly geared.

We step through this process in detail for specific markets in our Green Square and Mascot guides:

• /insights/refinancing-interest-only-green-square-safer-paths-forward
• /insights/refinancing-interest-only-mascot-loan-safer-paths-forward

If the revert amount is too high, you might:

• Refinance to a longer P&I term (back to 30 years).
• Move part of the loan to IO and part to P&I.
• Consider selling an underperforming property rather than stretching cashflow dangerously thin.

2.2 Your tax benefits from IO are weakening

From 2026–27, major tax changes reduce the long‑term value of negative gearing and the 50% CGT discount on many residential investments. That means:

• Running maximum IO purely for tax deductions becomes less compelling.
• Paying down non‑deductible home debt and maintaining only sensible investment gearing looks better.

If your IO investment loan was mainly about maximising deductions, it’s time to re‑run the numbers over a 5–10 year horizon. Our investment‑specific guide dives into this: /insights/interest-only-vs-principal-and-interest-investment-gearing-cashflow-tax.

2.3 You want more borrowing power in future

Banks generally like P&I:

• Lower ongoing risk than IO.
• Often slightly lower interest rates.
• Assessments assume you can handle principal repayments.

If you know you’ll need more credit later (upgrade home, buy another investment, fund a business), moving to P&I now can:

• Improve your profile in the bank’s eyes.
• Demonstrate a track record of higher repayments.
• Sometimes improve your borrowing power, especially if you switch from expensive IO to cheaper P&I.

2.4 You’re heading towards retirement

For pre‑retirees, a common target is:

• Clear home debt before retirement, or at least get it to a level that can be comfortably serviced on super and investment income.
• Retain only manageable investment gearing.

In your late 40s, 50s or early 60s, staying IO on your home loan while hoping for capital growth to bail you out is risky. P&I (possibly with an offset for flexibility) can be a better fit.