Wellington is a bit different in that there is a limit on the amount of empty land in the centre and the neighboroughing regions: it's quite hilly and pinned in geographically, and most of the central regions are fairly built-up already. Some areas (near the CBD) were also built up on reclaimed land from the harbour, and what with earthquakes, that's not great...
[1] http://eggcorns.lascribe.net/forum/viewtopic.php?id=7251
I think compliance costs and times don't help. Neither does our building code. We should allow the use of materials that have been approved in more advanced countries like Germany.
The real cause has been low interest rates. People borrow as much as they can. At sub 2% rates they can borrow a lot and are pushing prices up.
And if you build a house you're adding to the housing market (assuming it wasn't a teardown/rebuild) whereas if you buy it's just shuffling existing stock around.