If I had 18 days in Australia, I'd do 4 days in Sydney, 4 days in Melbourne, 3 days in Alice Springs (Ayers Rock/Uluru), 5 days in Far North Queensland (Cairns, Great Barrier Reef, Atherton Tablelands). That gives you some wiggle room to adjust as necessary, and/or to account for travel.
As far as time of year, it all depends on what you want to see/do. If you go up north to see the Great Barrier Reef (which really is a must), January-June is the "wet" season, where it's hot and muggy pretty much all day, every day. Winters in the south (June-August) are frigid/mild, but not freezing most days.
I would split the difference and go in the October range.
Sydney is fantastic, and you must go to the Blue Mountains. It's a lovely day trip. Also, for a cheap, spectacular view of the Harbour Bridge and Opera House, take the 30 minute Neutral Bay ferry from Circular Quay. It passes right past the Opera House and Harbour Bridge, and is a terrific way to do those two sights.
Melbourne is also great. Lots to see and do there, but don't forget to enjoy some Italian food on Lygon Street! If you go between March-October, make sure to catch an Australian Rules Football ("footy," at least in Victoria/South Australia) match at the Melbourne Cricket Ground (MCG). Just don't let anyone talk you into becoming a Collingwood or Carlton fan.
Uluru is pretty self-explanatory.
Far North Queensland all depends on what you want to do. Of course, snorkeling or diving the GBR is a must, but there are some absolutely wonderful areas around Cairns. The Atherton Tablelands, 30 km inland from Cairns, are an incredible mixture of lush tropical rainforest, fertile agricultural lands, and rich cultural history. There is a quaint town up on the Tablelands called Yungaburra, which is one of my absolute favorites, and well worth a visit. If you want a more coast-oriented town, Port Douglas, an hour north of Cairns, is a great beach town. Further north is the beautiful Cape Tribulation. Either way, make sure you take a cruise on the Daintree River, and if you're lucky, you'll get to see some crocodiles!
Personally, I'd fly CX CDG
, then jet around using the low cost carriers like Virgin and Jetstar, (book well in advance for the cheapest fares), then return CNS