Hi Tim

If your priority is windsurfing, my suggestion would be to start with three weeks in Western Australia. Perth can probably work ok for slalom but I would head up to Lancelin or Geraldton.
Here are some WA retailers: http://windsurfwa.com/WA/gear.html

Then fly over to Carins to do some diving on the reef for one week. If you are an experienced diver go for a overnight trip, the one day trips are often crowded and not as good. Pro Dive would be my choice: http://www.prodivecairns.com/
(If you have time also head up to Cape Tribulation, stunning!)

Then over to NZ! If you want to visit both islands and see as much as possible in 6 week here is my itinerary: Auckland - Bay of islands – Coromandel – Rotorua – Taupo - Taranaki – Wellington – Kaikoura – Christchurch – Wanaka - Queens town - Milford Sounds. Definitely enough time to squeeze in some great windsurfing!

Not sure about rentals in NZ but these retailers can probably help: http://www.windsurf.co.nz/windsurf_retailers.asp

Whatever you decide, you will have a great trip!
here are some photos for inspiration:
http://secretspot.se/gnaraloo/
http://secretspot.se/oz/

http://secretspot.se/new-zealand/
http://secretspot.se/nz02/

Cheers

... and when in NZ don't forget to stop by in Taranaki of course!

Just one thing that I want to know:

I need a car in Australia and in NZ, of course. I wanted to rent a camping car, but with the most camping cars it is difficult to take some windsurfing gear with me and my girl friend.

Any idea how I should do it?

Cheers
Tim