I sometimes hear pub talk to the effect "Let them prove it". If a taxpayer takes this attitude with tax law, he or she might have a problem.

The principle "innocent until proven guilty" does not always apply when it comes to tax law.  In some cases the law is "guilty until proven innocent". This was reinforced, not for the first time, in the case of Zappia v Commissioner of Taxation in a judgment issued in the Federal Court of Australia on 19 April 2017. $2 million was deposited into the account of the taxpayer on 30 June 2010. She did not include this amount as income for the 2010 year. The Commissioner issued a default assessment to the effect that the $2 million was income. The Judge found in favour of the Commissioner. Part of the judgment contained the following words (bold emphases added by me).

"The issue is not whether the Commissioner made an error nor, perhaps a little surprisingly, even whether the $2 million is assessable income. Rather, it is whether Mrs Zappia has proven that the $2 million is not assessable income. It is implicit in that statement, which is about who bears the burden of proof, that the Commissioner is under no obligation to prove that the $2 million was income. The onus lies, rather, on Mrs Zappia to prove that it was not.

The judgment is long but interesting if you enjoy a bit of forensic detective analysis. Here is the link [2017] FCA 390

There is an important lesson here.  if you have a dispute with the ATO, try to negotiate and settle before default assessments are issued because your task is going to be so much more difficult once that happens. Hoping the issue goes away rarely leads to the most favourable result.