Best time to go to australia?

Im planning a trip to go to australia sometime next year. I want to start at melbourne and work my way up the coast to cairns. I want nice weather that is suitable for swimming in sydney and scuba diving in cairns but i dont want to go when there are too many people. The best time for me would be june or july but i dont know if their winter is warm enough to be able to swim and scuba dive and be outside and stuff. If june and july are too cold, what could be some other times? ive heard september is nice but i don think i can go then