"police officers duped into believe that it's a legitimate operation because GCHQ ordered it"
New Zealand police didn't need an excuse to be gung ho. The corruption in that organisation runs from the top to the bottom (not so much "bent coppers", but the even worse Gene Hunt "Noble Cause" type of corruption(*)) when it comes to "getting a bad guy by any means possible", along with overly credulous belief of any foreign "expert" whose pronouncements align with their interests. ("We know he's a bad guy because XYZ said he is"). Being able to kick in doors and raid using helicopters and tactics normally only seen in hollywood films was just too much of an opportunity for the people involved to pass up.
The illegal raids they'd performed in other operations(**) along with evidence planting and manipulation(***) have sometimes(****) resulted in cases being overturned, but with the government pressuring the courts to "make it happen", the Dotcom case is the final straw in proving that NZ really is a Banana Dominion.
People may not take bribes in NZ (although this appears to be changing rapidly), but influence peddling, cronyism and power-based corrupt activities are rife (and usually not illegal under NZ law - bribery is pretty much the only kind of corruption which is explicitly illegal)
(***) https://en.wikipedia.org/wiki/List_of_miscarriage_of_justice_cases#New_Zealand is just the tip of the iceberg there.
(****) It is a brave judge who criticises the NZ Police, given the systemic retaliation that happens when they do - that's a (paraphrased) comment made by a NZ high court judge I knew in the early 1990s.