is it technically insolvent?, page-26

A couple of points

You are correct that ANZ would raise a provision for tax under self assessment which may or may not agree with the ATO assessment. However the last para of the announcement above states that due to the uncertainty of the particular issue an additional provision has been raised as a contingent.

Tax assessment by the taxpayer or the ATO is a lot of the time up to the interpretation of tax legislation. The ATO may not agree with the taxpayers interpretation but this does not mean the ATO is right and the taxpayer is wrong.

With the exception of the mass market tax schemes the ATO over the last few years has not had a good run in winning cases at the AAT or courts in cases of law interpretation.