For many diving in Australia offers a diving mecca for tens of thousands of tourists each year. The must-go place that is often spoken of is Cairns. It is here that the weather is warm enough for around-the-year diving to take place. The Great Barrier reef is the big attraction for many divers, it is a massive living coral organism that is reputably visible from space!
Australia is in the Southern Hemisphere and much of it is Tropical / Sub Tropical so seasonal weather much different to Northern Europes. Having said thatAny time is a good time to be in Australia. The Australian Summer (December to February) can get uncomfortably hot but it's great diving weather especially up on the Cairns region barrier reef. Up north, the summer wet season is very, very humid and the sea is swarming with box jellyfish. Winter (June to August) offers skiing in NSW, Victoria and sometimes Tasmania. In spring and autumn the weather is mild.
The Dive Scene in Australia is typically well governed, regulated and safety driven. The BSAC organisation (club) is a smaller and more isolated organisation here due to the lions share of dive operations being dominated by PADI which is a local powerhouse in Resort-based activities in Cairns.
So you've got Company and Club styles of diving but what about the divers who don't want to spend lots of cash on the Dive companies and/or live nowhere near a dive club. Well if you're already a diver you'll probably know this answer. Dive off your own resources. First you should be experienced enough to fully organise, plan and carry-out a days diving. You'll need all your own dive gear, plus an air cylinder (obviously!) and a means of getting out to the dive site which can vary from your own two legs to a fully fledged liveaboart dive cruiser! Unless you're a solo diver you'll need a buddy, the on-line forums/newsgroups/bulletin boards often have independent spirited divers posting messages for dive buddies/companions for diving trip. This option is often a purists favourite and there are benefits, especially if you and your buddy come from the same area as he/she can go halves on fuel costs!