Hi Ian,
I will chime in here (even though I now live in Oz, having moved here 3 years ago). I still spend a bit of time in NZ, and travel across once a year just to fly jets..
Ross is correct, Tokoroa is a fantastic site for jet flying, and is where most of the action in the upper North Island happens. The drive from Thames to Tokoroa would take you a bit less then 2 hours. The NZJMA has regular weekend flying there one weekend per month during our summer, and the local club fly props there every weekend.
Go to
http://www.nzjma.com/index.htm for further details. There is a calendar there as well as contact info. The kiwis are a great bunch, and will make you very welcome. Make contact with them via the site, and they will tell you everthing you need to know.
If you are going to be there in Feb, the Annual Trans Tasman Jet meeting is on 18,19,20 Feb, and is easily the biggest meet in NZ. (although very laid back by international standards). I travel across for it every year as it is a chance to get some good flying in at a great site with no pressure on the flight line.
Roger