Since you're flying to Perth, it's worth spending a day biking around Rottnest Island. Catch a ferry from Fremantle. Very pretty, and virtually car-free. Also, you might want to consider biking in the SW corner of Western Australia. I didn't have enough time to go there myself, but it sounds like a nice region for biking.

Definitely spend some time in Tasmania, which was by far, my favorite part of Oz for biking. I wouldn't cut back that much on Australia to catch an earlier boat to NZ, however. But in mid-summer, heat could be a problem on mainland Oz. I suspect you'll find it pretty comfortable compared to what you've been experiencing in SE Asia, however.

If you like architecture, don't miss Napier on the east coast of the North Island of NZ. It has the best collection of art deco buildings I've seen anywhere in the world.

While there is a lot of nice and varied scenery in NZ, I think 4 months might be a bit too long. If you enjoy hiking in addition to cycling, the national parks in NZ are wonderful.

BTW, I really like your website. You've put lots of useful information in it for the various countries you've toured in, and your website is pleasant on the eyes.