If you are in northern Queensland then definitely give the Great Barrier Reef a go.
But if you want to get away from the tropical thing...
If in New South Wales, i've heard Byron Bay is great, Sydney has some interesting dives, and south of Sydney Jervis Bay is pretty good. Around Sydney there's a few shipwrecks that can be freedived. The only one i've done is the Valiant, off the northern beaches (Palm Beach specifically).It sits on a sand bed at about 26m. ADR from this forum posted some photos in the OzFreediving forum a while back at
www.ozfreediving.com.
A navy boat was scuttled off the Queensland coast a few months ago - it's open for divers. I believe it's off the Sunshine Coast (maybe Mooloolaba). Contact the Brisbane City Council as I think they administer that sort of thing.
Also go to these forums to ask around-
www.extremespearfishing.com.au (good range of QLD and NSW divers)
www.spearfishing.com.au (mostly QLDbased)
www.spearfishing.net.au (good range of QLD and NSW divers)
These websites are spearfishing based but have a few resident freedivers hanging around. Ozfreediving is the only one dedicated to freediving.
What time of year are you planning?
Cheers,
Ben