Hi Paul
You need to find your local panel doctor which can be found on this website
www.immigration.govt.nz would then suggest contacting your nearest panel doctors and ask for the availabilty dates and prices
We had to wait three weeks for available date and we live in the same town as the doctor(the panel doctor may have to cover a large radius of surrounding towns), apparently they are now having up to fourty a week for Australia, Canada and New Zealand.Ask if they can post you a price list.We also had to pay for chest X-Rays which we thought was in with the price of the medicals.
The results where available one week later.I believe they are valid for three months.
I guess it depends on when you plan to move to New Zealand
D