Hi Paul,
Having travelled out twice to NZ from the UK recently, I can give you some info on my experiences. However, Sean & John live in NZ so hopefully if they see this thread they will be able to provide you with even better info!
I found standard b&w film a little more expensive than in the UK, with the exception of my favourite film Kodak infrared which was cheaper! The best shop I found for price and a excellent range of materials was was Photo & Video International in Christchurch
http://www.photo.co.nz/ I was able to order my films before I went, and collected them on arrival. They also will send stuff out, e.g. if your student was picking up a campervan or whatever, they will send the order to the pickup point (as long as it is paid for in advance). They will also negotiate on price if your order is large

Photography shops in Dunedin carried a range of black and white materials but it was much more limited and the prices were higher.
As far as the X-ray are concerned, before I went I discussed this at length with the technical department at Kodak who informed me that scanners (for hand held luggage) at modern airports were pretty much safe as far as they were aware. (They cannot be absolutely precise as for security purposes airports don't give out specifics on scanning). Having said that, Kodak advised that each pass through the scanners has a some affect on the film and this is obviously cumulative, so by the time the film has received 5 passes it starts to show more significant effects. In practice if the film is below 800 ASA it is minor but worth considering.
I took the view that I would prefer to buy my film out in NZ and so reduce the number of passes through the scanners to just the return journey.
At Christchurch Airport they were happy to carry out manual searches of the film, but as most of mine was infrared film this would have been a disaster so I let it go through the scanners...all my film was fine.
Hope that helps...
Kathy
