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Interest Deductibility: Splitting Investment and Personal Debt

If you own both an owner-occupied home and an investment property, understanding debt deductibility is crucial. The interest you pay on each is treated differently by the ATO, and structuring your debt correctly can save thousands in taxes.

Basic rule:

Interest on a mortgage for your principal residence is NOT tax-deductible.

Interest on a mortgage for an investment property IS tax-deductible.

This means: if you own a AUD 700,000 investment property with a AUD 600,000 loan at 4.5%, you can deduct AUD 27,000 in interest annually. If you own a AUD 600,000 home you live in with a AUD 500,000 loan at 4.5%, you cannot deduct the AUD 22,500 in interest.

Where it gets tricky:

Scenario: you own a AUD 400,000 investment property with AUD 280,000 debt. You also own a AUD 600,000 primary residence with AUD 450,000 debt. You want to refinance to get a better rate.

Option A: refinance both mortgages into one loan with a single lender.

The lender gives you one AUD 730,000 loan (AUD 280,000 for investment + AUD 450,000 for personal).

But the ATO requires you to split the interest: AUD 280,000 of interest is deductible (investment property), AUD 450,000 is not (personal residence).

You must track this carefully and claim only the investment portion on your tax return. Many people get this wrong and claim interest they’re not entitled to.

Option B: keep two separate loans.

This is cleaner: one loan for the investment property (interest fully deductible), one for the personal residence (interest not deductible). You know exactly which interest is deductible.

The downside: two loans mean two sets of fees and possibly two different interest rates.

Best practice if you own both:

  1. Refinance the investment property with a separate lender using the existing loan
  2. Keep the personal residence loan separate
  3. This way, you have clear delineation and no risk of claiming non-deductible interest

If you must combine loans:

  1. Have the loan agreement explicitly split the portions (e.g., “AUD 280,000 for investment property, AUD 450,000 for personal residence”)
  2. Track interest calculations for each portion separately
  3. Claim only the investment portion as a deduction
  4. Keep detailed records and provide the loan agreement to your accountant

Real-world example:

Investment property AUD 280,000 loan at 4.5%: AUD 12,600 interest/year (fully deductible) Personal residence AUD 450,000 loan at 4.5%: AUD 20,250 interest/year (not deductible)

If you refinance into one AUD 730,000 loan at 4.5%: Total interest: AUD 32,850/year

You can claim: AUD 12,600 (investment portion) You cannot claim: AUD 20,250 (personal portion)

If you forget to split and claim AUD 32,850: The ATO might challenge you and deny the AUD 20,250 deduction, plus penalty and interest.

Another scenario: what if you’re paying down a combined loan and want to know which portion to pay off first?

If interest rates are identical, it doesn’t matter mathematically. But strategically, some investors prefer to pay down the non-deductible debt (personal residence) to reduce non-deductible interest.

Tax planning opportunity:

If you have substantial non-deductible debt on your personal residence and investment property equity, consider:

  1. Refinancing the investment property to pull out equity (investment debt)
  2. Using that equity to pay down the personal residence mortgage
  3. This shifts debt from non-deductible (personal) to deductible (investment), increasing deductions

This is called debt conversion and requires careful structuring with your accountant.

The bottom line:

If you own both investment and personal property, keep debt clearly separated and track deductibility carefully. The tax savings are significant if you get it right, and the penalties are steep if you don’t.

Work with an accountant familiar with property investment. They’ll ensure you’re claiming everything you’re entitled to and not claiming what you’re not.