"Dental Plaster" is a very vague term, and covers everything from just barely stronger than the 2000 psi crush strength of Plaster of Paris to 18,000 psi modified plasters. I think you need a bit more information before buying. There have been several topics (actually, a LOT of them) about the availability of plaster in various countries, such as Australia, but I don't recall seeing NZ mentioned. You might want to check through the older posts here on the forum and see if there's anything listed.