The reverse onus is the rule that shifts the burden of proof from the taxpayer to the Minister of National Revenue. In almost every tax appeal the taxpayer must prove the assessment wrong, but for a gross negligence penalty under subsection 163(2) of the Income Tax Act, subsection 163(3) places the burden on the Minister to establish the facts justifying the penalty.
At the Tax Court of Canada this means the Crown has to prove that the taxpayer's conduct met the high threshold for gross negligence, rather than the taxpayer having to disprove it. The reverse onus is the strategic centre of any gross-negligence appeal and a frequent reason such penalties are defeated even where some additional tax is upheld.