Visit this link –
https://www.police.govt.nz/advice/firearms-and-safety/changes-firearms-law-prohibited-firearms/firearms-changes-faqs
So.
You can potentially pin a .22 with more than 10 round capacity, or shotgun with more than 5. You can also cut the tube down.
However – it needs to be a ‘permanent’ modification – so the word ‘pin’ is not really the correct one to be using. I would suggest a pin, or something easy removable will get in hot water.
As, I note, likely to cause a few headaches – is putting more than 5 short shotgun rounds into a tube that can only hold five ‘breach length’ cartridges. I.e. don’t play those games – it always makes me nervous when I read something that includes the words ‘the intent is’ – as that likely means the specific regulation/law has enough leeway within it to be interpretable. You don’t want to get into an argument with a cop around firearms on interpretation. It won’t work out well.