A lot of demand for housing (both to buy and rent), and in my experience (live in Wellington) a lot of the newer apartments being built seem very much architected with buy to let in mind, so I think renting is a large part of it, along with limited supply.

The government did tighten the rules a bit several years ago so that effectively (there are ways around it), you needed to be a resident to buy property, but I don't think that had that much impact (although maybe it would be even worse if that wasn't done?)