can confirm there are lots of guns in NZ.. i was in the scene for a while and can confirm this.. have held some rather nice examples from colt 32 special to a vintage Webley mk6, a working Luger (that i really wanted to buy off the guy but had a feeling it had been used) a really rare ruger 22 with built in silencer, several 1911's and that's not including the modified's (sawn off double barrels, cut down rifles, I've even had a look at a rather well made pen gun) then there's the legal ones, a Mossberg 500 (the one with the folding stock you usually see in movies ex-IDF surplus) the pump action shotguns you can legally order and import.. make no mistake just because handguns guns are illegal in NZ doesn't mean they aren't out there and in use by the guys who would most of the time use them to protect themselves from getting ganked but wont hesitate to use it to intimidate.. I'm also not saying every crim out there has them but with some of the characters I've had the misfortune to meet the popular saying was "Better to have it and not need it than Need it and not have it"
its a gamble when you associate with those circles, that's why its always good to have a friend that's on the fringe of that.. a dodgy cousin or a friend that knows a guy..someone you have a certain level of history with and therefor trust.
going to the cops will surely get you at the very least looked into.. if not your past, then certainly your future. (wont change much, just when you get pulled over on the side of the road it becomes a difference of "got anything illegal in your car/on your person tonight?" to "under section 18a of the misuse of drugs act I'm gonna search your car for drugs" but like others have said the cops will be more interested in getting the gun off the streets and the guy who's using it, you'll just get a lecture (and laughed at behind closed doors) there was a bloke several years ago on that police show that was growing 2-3 plants in his garage and was ripped off, he called the cops and reported himself and the guys that stole it.. not much happened to him just a slap on the wrist iirc, who knows these days..