Chch seems to be the best location for this. Beware though, it's not for the faint of heart!
Most other places seem to have a few reg girls, usually with addiction issues.
I reviewed one recently.
Plot twist: The girl I booked didn't want to take the booking so B&Sed me with her (similar-looking) flatmate, - who was actually pretty good!
I think your probably right about Christchurch, I'm Wellington based and have had a handful of good and well priced ones in the past 7,8,9 or so years and a few good Auckland one's but the best market always seems to be Christchurch