Retroactive legislation

The comfort that so many on the left of the political spectrum have with Parliament legalising a past event when it was illegal is curious, and has everything to do with the knuckle dragging tribalism of two party politics. Make no mistake, there are plenty on the right who would take a similar approach if this had happened to them.
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However, if we look around the world we’ll find while this is accepted practice in Westminster style parliamentary democracies, it is not so accepted in constitutional democracies. Certainly in terms of criminal laws, few countries tolerate retroactively making actions or omissions criminal. Even in the UK, the European Convention on Human Rights binds Parliament on this.
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So how does the NZ legislation meet the test internationally?
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The US Constitution states in Sections 9 and 10 that Congress and State Legislatures are prohibited from instituting ex post facto laws. So that’s it, it would likely be unconstitutional in the USA.
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Sweden’s constitution allows retroactive non-criminal legislation that can only apply from the date that the bill was proposed by government. Hardly applicable given the breach was before the bill was proposed.
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France’s constitution only prohibits retroactive non-criminal legislation (is silent on this but explicitly prohibits retroactive criminal legislation unless it benefits the accused). So France may allow it.
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Norway’s constitution prohibits any laws from having retroactive force.
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Canada’s constitution only prohibits retroactive criminal laws.
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So you can see, it is a patchwork. However, the real reason there is compliance about this law, mainly from Labour, NZ First and United Future supporters (both of them), is tribalism in politics and NZ has now entered the most tribal phase in party politics that it has seen in some years. National sees power close and Labour is anxious to hold onto it.