I know after two years of cycling you may not want to linger too long down under or any longer in Asia .... but you are arriving in Australia at the worst time to be cycling ... the hottest time of year and exactly when every man and his dog will also be on summer holidays. Accommdation, campgrounds and road traffic will all be at maximum capacity.
I would extend in Asia, maybe do sub-tropical China or head over to Borneo to see the Orangutangs, or ride down Malaysia to Singapore then arrive into Perth around late Feb or March. School goes back at the end of January, and university starts up in early March. Spend 6 months in Australia (do the south in late summer/autumn then go North for the winter: Fraser Island, Whitsundays, Hinchinbrook Island, the Daintree, Atherton Tableland, Great Barrier Reef - endless possibilities!) and head over to New Zealand for spring (still cold!) and summer.
Or go to NZ 1st and back track to AUS if the costs are not prohibitive.
If you don't want to delay getting back to Canada and your timeframe is still set for arriving in Australia around Christmas here are some suggestions:
(With the following weather disclaimer: You may be lucky with the weather, but last year South Australia had a shocking summer, with a record number of days over 100F and even Hobart at the bottom of Tasmania had at least one day well over 100F. Then again it might snow around Cradle Mountain in the middle of summer! All of the southern part of Australia was hotter than the east coast last summer.)
For Western Australia definitely the south west corner, Margaret River region and the stunning Karri forests. The WA gov has built some excellent bike paths to get you out of Perth and down south.
There is an excellent bike trail (gravel) but I'm not sure if the track will be suitable for your touring bikes. Its the Munda Biddi,
http://www.mundabiddi.org.au/ , and has the following facilities:
"Purpose-built cycle-friendly campsites have been built every 35-40km between towns. Campsites have a composting toilet, two water tanks, picnic tables, undercover bike storage facility, sleeping quarters for 25-30 people, and cleared tent sites. " National Geographic listed it as a world top 10 cycle trail. I can't comment on the accuracy of the ranking! :-)
Also ask at the WA Cycle Touring association for good places to cycle and whether you could do all, or sections of, the Munda Biddi:
http://www.ctawa.asn.au/
You have made a very good call to take the train across the Nullabor!
In South Australia there is some outstanding cycling out towards the Flinders Ranges and Wilpena Pound - again on a dedicated cycle track, but I think it would be too hot at that time of year. Google Kangaroo Island - that would be terrific cycling and has outstanding wildlife (its just about a must do!). You could cycle down the Fleurieu Peninsula (south of Adelaide) dropping by the Mclaren Vale wineries and then ferry across to KI. If you have time you might like to cycle through the Adelaide Hills to the Barossa Valley and/or Clare Valley, the wine growing regions north of Adelaide.
In Victoria the Great Ocean Road, the Grampians and Otway National Park are on my cycling to do list. Anywhere along Victoria's southern coast would be terrific. I ditto the idea of catching the ferry across from Queenscliff to Sorrento once you get to Victoria.
All of Tasmania should be terrific. Try to do the wilderness areas in the west as well as the more trodden paths.
If you extend your trip, I know most cycle tourists take (or recommend) the inland, mountain route from Melbourne to Sydney (or vice versa), but we are planning the coastal route. The south coast of NSW and north east Victoria is terrific, has heaps of wilderness, wildlife and National Parks. Loads of free camping opportunities!
If you "cruise" from Adelaide to NZ, Bass Straight can be absolutely shocking, and the Tasman Sea made even Josie Dew sea sick, so don't forget the sea sickness preventatives. Josie's book on cycling New Zealand is really good, and has an outline map of her tour there. Josie went by cargo ship from Europe to NZ then from NZ to Melbourne.
BTW Lonely Planet are releasing 4 new editions of their Australian guide books in October: Tasmania 5th edition, QLD & the Great Barrier Reef 5th ed, East Coast Australia 3rd ed, and Melbourne & Victoria 7th ed. You could download the portions you need or get free info from TICs.
Ok, I could write a book, better stop here!
Just ask if you need specific information instead of ramblings!