4 months in Oz and NZ, is that fair?

[canadianbunny]

g'day punks,
I'm working on planning my RTW trip, and it worked out that Oz and NZ would be squished into 4 months... I'm assuming 2 months in each country is fair. But will I do them justice?

I've been to Oz in 2007, but I traveled along the sunshine coast (Cairns - Sydney). I love Aussies, so I just have to go back!! Aus and NZ are the most expensive countries on my travels, so I'm looking into WWOOFING.

Would it be realistic to buy a crappy used car and try and drive around Aussie- say follow HWY 1 or something? Or is this a bad way to see Oz on a budget? I met some Americans last time I was there, and they had bought a used car to drive around Oz in. Can I give Oz justice in only 2 months?

As for New Zealand, I met one Kiwi in Oz, who was awesome- now I know NZ is much smaller then Oz, but has sooo much to see and do- is 2 months fair for NZ? I haven't been to NZ yet, but blackwater rafting, zorbing, bungee jumping, sheep... I must see it all!

Is the cheapest/only way to get from NZ to OZ by plane?