chevrolux: If you can find someone local to install and Asterisk box that would be a bit cheaper. But labour will be a bit higher compared to a branded PABX as there is a lot more set up to do. All you have to pay for is the IP phones and the server hardware so maybe if you have a supply deal with the distributors (Ingram Micro, Snapper Net come to mind first off) then it could be a good option. Then it is just a case of getting a guy (or girl, better be PC) to set up Asterisk.
This is the other option I'd suggest. I've sent you a PM - let me know if you're keen for someone on the ground in Tauranga to help out with this project.