I'll throw my two cents in, since we sold a lot of VOIP lines to people in your market (1-3 lines.)
We used WorldxChange, their platform is very good, and they are very focused on security and having a good network, although they have had some unfortunate problems which seem to be behind them. (They purchased some very expensive kit which the manufacturer had firmware problems with and it kept bringing down their network - not a nice problem and was not because of underinvestment - exactly the opposite.)
Their home VFX service is very cheap, works very very well but only be connected to with approved devices so they can control the quality and consistency of the service. Because of this the help desk can fix any problems with your config and push down config changes to your device to fix any problems. A more 'managed' service for people who just need VOIP to work with the savings associated with it.
2Talk is the other end of the spectrum, they say bring whatever SIP device you want and plug it in - here's some documentation away you go. Their web interface is quite nice, but SIP is a complex protocol when you factor in all the millions of devices and interactions between them all, e.g. just getting the number of rings to be x across all devices can be very hard and then factor in that you can take any device to plug it in and it won't ever work as 'standard' or as reliably as a VFX line - but there's that freedom or bringing in any device and away you go.
Orcon is the only other provider I really know of which offers SIP lines, but I'm not sure if they do a one off connections, but I've never heard problems with their SIP Server/Telephone exchange going down or having problems.
Then there is the skype (not SIP based) which I believe you can get phone numbers for NZ for, and you can get apps for your phone (e.g. iPhone) so you can have your landline wherever you have a wifi connection. But be warned the pricing is not cheap compared to the NZ providers for NZ calls.