Retrospective Law is a bad thing and undermines confidence in Govt decision-making. Society at large does not have the same ability (I can't just tell the IRD I screwed up a tax return, but hey, I've decided it's no biggie) to walk away from their errors in life. Previous examples, such as legalising election over-spending, were extremely cynical abuses of power. In that particular case, they were at least 'self-serving' - in as much as they were effectively 'administrative' issues that would have warranted SFO attention, but only just.
Retrospectively legalising a lockdown of citizens in their homes that vapourised billions of dollars from the economy is next level. Sure, it's not retrospectively legalising ethnic cleansing, but if the state can walk past the law this far without fear of consequence, what reason do they still have to be accountable to the societies that they are elected by to represent them?