we are just 2.2 km off the Pacific Hwy, half way between Sydney and Brisbane, you are welcome to a bed and hot shower if you pass by ... if we are not off touring ourselves! We've got a very quiet 60 acres to pitch your tent if you prefer, and about 100 kangaroos hopping around for an up close and personal experience! :-)
If you ride between Melbourne and Brisbane at either of those times you have chosen there is nowhere along the coastal route that you will experience 50 degree heat. In land in northern Australia would be horrific though. My son lives in Katherine, NT and cycle commutes to work each day on a cycle path and is horrified at the idea that we plan to cycle up to visit.
Late spring/early summer you may experience some wild summer storms in the afternoons along the NSW coast. However what may be worse is that late summer through autumn is our wet season and you may get prolonged rain along the coast. Having said that you wouldn't get a better climate anywhere in the world than the north coast of NSW and through SE QLD during autumn. Beautiful warm days and mild overnight. In fact our winter days are perfect for cycling, average low to mid 20's C.
Generally the cycle tourists (and grey nomads) start coming through here around March in an effort to reach western QLD and the NT in winter. The grey nomads wait until the summer holiday traffic is off the roads. Cycle touring during late December to late January would be awful given the volume of summer school holiday traffic on Australia's highways which are not all that bicycle friendly at the best of times.
Some Aussies will recommend that you stay further south during summer, but we watch the weather charts daily and to be honest for the past two summers Victoria and South Australia have been 10 degrees hotter through summer than here on the North Coast. In fact we thought it was hilarious, because a friend of ours migrated from England to Grafton and after 2 summers she moved to South Australia 2 years ago for the better climate!!! It is SOOOO much hotter and dryer down there, especially for the past 2 summers that she has been down there.
There are heaps of good blogs on cycling the Brisbane to Darwin route, not as much perhaps on the Melbourne to Brisbane route. One thing I have found is that a lot of cyclists on the Sydney to Brisbane route rush along and miss the all the best things to see!
Also when getting out of Sydney, consider missing the highway and taking the scenic route by ferry. You can ferry from the city to Manly, cycle through Sydney's northern beach suburbs to Palm Beach and take the ferry to the central coast. There is also a hostel in the national park on the western side of Pittwater (opposite Bayview). You can cycle in through the national park via West Head. The views over Pittwater are absolutely stunning. (The bike path into the hostel from West Head might be more mountain bike territory though) however you can catch the ferry across from the eastern side of Pittwater. That would make a nice diversion for the day and a nice place to stay. Otherwise you could camp at Narrabeen Lakes halfway between Manly and Palm Beach or alternatively do the Sydney to Central Coast in 1 day and camp on the northern side of Pittwater the 1st night out.
Get off the highway a bit coming up the coast otherwise you won't see anything much worth seeing. If you check out David in Oz's blog he is currently circumnavigating Australia. He started in Perth and came via Sydney/Brisbane/Cairns and is now somewhere in the NT. He detoured off the highway a lot and camped in some nice places. eg Diamond Head.
There are 2,000 yo Aboriginal stone fish traps in the Ocean at Arrawarra just off the highway a few km between Grafton and Coffs Harbour, and a small but interesting Aboriginal musuem and bush tucker walking trail. Its not something that tourists ever go and see, but if you are passing by its an interesting diversion if you re interested in Australia's indiginous culture.
+1 for Dorrigo National Park.
In our immediate area Red Rock, Diggers Camp, Minnie Water, Angourie or Woody Head are terrific places to spend a night if you want beach and bush. If you need further recommendation the billionaire founder of Billabong is building a house at Angourie.
If you prefer to miss the far north coast you can take the back road from Coffs Harbour to Brisbane via the Summerland Way, though there are some big hills over the mountains on the border. Or cut across from Kyogle to Murwillumbah through Uki (Mt Warning National Park at Uki is a must if you go that way) ... absolutely beautiful! If you want to get to Byron Bay, you can still do that from the back road. There is a steep, gravel road, (very scenic) with very little traffic, from Uki to Mullumbimby, then its a short ride to Byron Bay.
Alternatively from Murwillumbah take the back road through the Gold Coast hinterland. Very lush and beautiful National Parks through there. By the time you tour I hope to have a web site available with maps and directions for these Pacific Highway alternatives and links to relevant touring blogs.
There is a new book available for cycling South East QLD. I don't have it (yet) but the marketing blurb looks positive.
best regards