Public law (or restitutionary) remedies are usually relied on as a last resort by taxpayers facing CRA assessments. They are last resorts because they are only available in exceptional circumstances, and the CRA almost never applies them, while the Courts rarely apply them.
One interesting historic restitutionary remedy, first established by the Supreme Court in Kingstreet Investments Ltd. v. New Brunswick (Finance)– and now called the “Kingstreet” remedy – allows a taxpayer the right to recover the taxes levied under unconstitutional legislation which before Kingstreet was doomed to fail under a claim for unjust enrichment against Crown.
The Federal Court in Canadian Pacific Railway Company v. Canada (“CPRC”) had the opportunity to consider this special remedy, and underlines its limited application: only being triggered when a tax charged by a government is constitutionally ultra vires (i.e., by virtue of unlawful legislation), and not triggered because of some unlawful government administrative actions!