Sabbatical in Australia or New Zealand

Hey everybody,
I am currently planning my sabbatical, which I want to start probably in fall next year (so there is still a lot of time for planning..). I am not sure yet whether I want to stay just in Australia or New Zealand, but probably I will mainly stay in Australia but also do trips to New Zealand.

I’ve never been to Australia or New Zealand, so I wanted to see a lot of the countries, learn new things but also do something useful..

I’d be grateful for any advice on the following topics:

1. Do you know any good associations to do volunteer work for?
2. Do you have experience with adult education centers that offer good language courses? I’d like to learn a bit of Maori, out of interest.
3. Is there a good way of traveling around a lot? Good train route, bus travels, renting a car, travel groups.. what would you recommend?
4. Any other tips, must sees?

Cheers,
Kenzo