Negative gearing is a very controversial issue. The latest round of debate stems from the Reserve Bank’s submission to the House of Representative Standing Committee on Economics regarding the enquiry into home ownership. The Bank believed that “there is a case for reviewing negative gearing”.

Treasurer Joe Hockey promptly responded to again rule out any change of the tax policy on negative gearing. In particular, it was reported that he claimed that removing negative gearing would create “an exception to a standing rule in taxation law”.

Is negative gearing in accordance with well-established tax rules? A fundamental principle in the tax law is that a taxpayer should be able to deduct expenses only if the expenses have been incurred to generate assessable income.

This is why an employee can only deduct expenses that are sufficiently related to work. For example, a funeral director at tropical Queensland would be able to deduct the cost of his black jacket (but not his black trousers) because the ATO believes that no rational person – except a funeral director – would wear a black jacket in such a hot place.

Should mortgage interest on an investment property be deductible? Investment properties generate two kinds of income: rental income and capital gains (if any). As capital gains on investment property can enjoy a 50% tax discount if the property has been held for at least a year, strictly speaking only 50% of the interest expenses related to the capital gain should be deductible.

In practice, it is impossible to predict whether there will eventually be a capital gain, and also impossible to predict the amount of the gain. This presents the key difficulty in the design of the tax policy on negative gearing.

Allowing full deduction of the interest expenses every year effectively allows deduction of expenses that may be incurred to generate the tax free portion of the capital gain (if any), and therefore may violate the fundamental tax principle for deductions. However, how can the Australian Tax Office (ATO) determine how much of the interest expenses should be disallowed every year before the investment property is actually sold?

Many countries resolve this issue by quarantining losses on investment properties. It means that losses generated from negative gearing cannot be used to offset against other sources of income, for example, salaries or business income. Instead, the losses can be carried forward to future years to offset against income from the investment properties.

This policy is fair in the sense that the same tax principle for deductions applies to both taxpayers with and without negative gearing. Many countries adopt this quarantine policy, including major developed countries such as the US, the UK, France and Japan.

Some countries have even stricter tax rules on investment properties. For example, China allows a fixed 20% deduction of the rental income, and the Netherlands tax property investors on a deemed yield rate of 4% on the value of the properties. In other words, these countries do not allow deduction of any tax losses on investment properties at all.

Australia’s current tax policy on negative gearing seems overly generous when compared to both groups of countries. More importantly, it is not fair to taxpayers who do not have negative gearing. The policy effectively subsidies negative geared property investors through the tax system.

The Reserve Bank concluded its submission to the enquiry into home ownership by rightly stating that “policy should not unduly advantage property investors at the expense of prospective owner-occupier home buyers … tax and regulatory frameworks should avoid encouraging over-leveraging into property”.

Of course, the negative gearing issue is complex and highly political. The ATO’s Tax Statistics for 2012-13 showed that taxpayers claimed a total of $12 billion tax losses from investment properties, and almost 1.3 million taxpayers had negative gearing.

The sheer number of taxpayers currently enjoying the benefit of negative gearing dictates that it will demand strong political will and leadership before a politician is willing to propose changes to the current tax rule on negative gearing.

Even if the government is bold enough to change the rule, careful consideration has to be given to transition rules to cater for the existing taxpayers that have negative gearing. This is a story for another day.