I'm not sure of many places but we were in the Bay of Islands (near the top of the North Island) last year and there were some cats for hire from the beach at Russell, which is the oldest town in New Zealand and the site of the first New Zealand Parliament. Can't remember what they were but from memory they were Prindles/Buffalo/Windrushes, probably about 14' long.
If you know where you are going there is a website
http://www.hobiecat.org.nz/ which is the NZ hobie cat association website which might be of some help, with listings of all the clubs around New Zealand. Contacts at each clubn will be able to help you.
There's several of us Kiwi's in the board, one in Marlborough (top of the South Island - beautiful place - do visit it), another in Gisborne (that's me), and I have noticed one or two others. Call in if you are coming to Gisborne - a long way from anywhere but nice and remote and one of the hottest parts in New Zealand (not always but often).