Interesting you describe the NZ market as idiosyncratic - I'm guessing it's not like European or North American cities then? I didn't get a chance to go to any tech meetups or such when I was last there (2017) so couldn't get a "feel" for what the market might be like.
I don't have any .NET experience but am using Java in my current job and am comfortable with it. 75% salary sounds reasonable considering the lifestyle tradeoff, I'd been figuring 50% as most of the software development jobs on trademe.co.nz at a glance seemed to be distributed between NZD$60k to NZD$120k, tops. Things like internet and food seem overly expensive in NZ compared to London though and rent isn't noticeably cheaper either, so I'd guess even a 25% pay cut may incur a bigger lifestyle dent than it would in London.
EDIT: my skillset in terms of languages is roughly, in order of familiarity: Python, Java, Go, Javascript. I used to be a Django developer and now do more general backend work.