In 2001, the United States taxed their population to the tune of 2.11 trillion. Of that 345 billion was uncollected, with 244 billion of it being individual income tax. This is known as the "tax gap" and is defined by the IRS as; "the concept of the tax gap as a way to gauge taxpayers' compliance with their federal tax obligations. The tax gap measures the extent to which taxpayers do not file their tax returns and pay the correct tax on time." I could not find much in the way of data on tax compliance in Australia.

Searches on the Australian Tax Office website for 'uncollected', 'unpayed' etc did not turn anything up. Nor did searching on the web under those terms. About all I found was the Victorian Auditor-General's office report which looked on written off taxes between 1993 and 1997.

It looks that the amount of uncollected tax is residually between 5 and 20%. This can mean that many of the government revenue claims are idealised. For a chronic deficit nation like the US has become this makes deficit spending doubly dishonest, as the 345 billion that is uncollected should be taken into account in appropriation bills.

In 2006-2007 the Australian government's revenues are projected at 217 billion AUD . It isn't clear whether GST is included in those figures or not. If there is a residual uncollected aspect to tax revenues - which any complex regulatory system inherently has - then it could be approximately between 10 and 40 billion by the Victorian Auditor-General's and US federal government's experiences.

Much of the US uncollected came from the self-employed. From the Washington Post article "Falling into the tax gap" [no link];

Most of that was due to unreported or underreported income - from self-employed people who did not have employers who filed W-2 forms to the IRS, for instance - or to the simple failure of some people to file tax returns.

This suggests that the regulatory overhead of the tax system is difficult to comply with without professionals or a dedicated office for those issues that larger companies can afford.

The lack of easily accessible data on this from the Commonwealth and State governments in Australia leaves the question open as to how much the Australian Tax Office is unable to collect.

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