I've managed to register by post (they emailed me the forms, I filled them out along with a photocopy of my passport and visa, and posted back), but I do need to go in to show them the real passport/visa before they will actually let me see the Dr on Monday. This was after a little bit of back and forth with me pointing out that I work in Wellington and live in Lower Hutt, and the surgery opens half an hour after I start work and closes about 20 minutes before I get home, and I didn't really want to take time off just to register! She did accept that as being a valid reason
For those in the Hutt area, a nice page on prices and individual GP contact details (which I found difficult to find on the Valley PHO website - a totally evil site to navigate, particularly in a crisis!) is here:
http://www.kowhaihealth.org.nz/valleypho.htm