^ may i ask how long you're planning to stay? thats a lot to cover properly if you're only staying a week or two
anyway i live in sydney so here are my recs for sydney:
- the rocks, sydney opera house, harbour bridge, sydney city (such as qvb, chinatown, darling harbour). you can walk to these places as they are fairly close to each other and if you don't mind walking in general!
- sydney fishmarket is a must for seafood lovers
- taronga zoo at mosman
- bondi beach (quick train and bus ride!)
- blue mountains, newcastle, hunter valley (vineyards) or wollongong if you're looking for an out-of-town or daytrip
victoria (i've only been once for about a week)
- great ocean road and the 12 apostles (you can do a day tour)
- phillip island for the penguins
- melbourne (capital city of victoria) has a great shopping and food!
- yarra valley for the wineries and food
- gold coast for the themeparks and beaches
- cairns and the whitesundays for the beaches/snorkelling etc but this can be fairly expensive depending on the resorts u go to
never been to the northern territory but one day would like to go
btw, it's autumn in april-may. sometimes it's a little warm or a little cold depending but i'm cold if it gets under 20 degrees and need to bundle up regardless due to my filipino blood so i'd bring some jackets/warm clothes if you plan to come around that time!
hope this helps!