500€ sounds more exie than a plane from Melb to Christchurch - are you sure you'll be saving money?

Definitely head south of Perth - the Margaret River region and the tall timber country is great. If you surf, WA has fantastic (but BIG) surf. I would prefer to spend time in WA rather than Gippsland (southeast Vic) if time is tight.

Re Victoria - the Grampians are nice. You could head north from the Great Ocean Road if you want. Western Victoria inland is pretty flat, there are some interesting desert parks and some wine areas as you get to the central highlands but I'd not bother.

If you take the great ocean road, when you get to Geelong don't head directly to Melbourne. Instead take the ferry from Queenscliff to Sorrento and ride up the east side of the bay - much nicer.

From Melb, rather than riding, you could take a train to Wangaratta and do a 4-5 day ride around there. Beechworth is nice. Bright is a pretty town (better in Autumn but that won't suit you), the area around Rutherglen has good wineries (famous for muscat and tokay fortifieds) or in the other direction you could ride the King Valley to Whitfield then through the hills to Mansfield (fewer tourist attractions but nice countryside).

I'll try to give you a few more ideas for Victoria when I have more time. Rowan, if you read this, any comments about central Vic eg the North or east then south loops from Wang?