For tassie most tourists dive 7mm wetties, which can be hired from all shops. Water temps in December can be around 12-15 C. If you are only doing 1-2 dives then its not worth the effort to go dry.
I'd second Steves recommendation of Eaglehawk Dive Centre. A top notch operation. Its one hour drive out of hobart but has some of tassies best diving. Several of their sites (under the mooring at Cathredral and Dragon City) have resident sea dragons. There are also some great caves, walls and, depending on season, a small kelp forest. If you overnight there then you'll find plenty else to do there such as visit Port Arthur and various scenic look outs.
Theres several shops in hobart that offer trips: but although hobart has plenty of nice shore dives its best to head out to eaglehawk and dive there.
Further up the coast is Bicheno Dive centre which has excellent deep (>30 m) diving - but I would only recommend it if you are fully comfortable diving cold water first. Ditto Bay of Fires dive centre at Binalong Bay up the far north east of the state: a very scenic spot to visit.
Haven’t gone out to the Troy D with Mick yet - look forward to doing so next time I get down there. The Isle de Phoques should be good for seals - though personally I've avoided diving in seal colonies.
In the back of my memory there may be a diveable seal colony in/near Melbourne's Port Phillip Bay?
For Brisbane - well although I live here you shouldn't ask me: I despise coral, and I hate having to dive wet...

Temps are way too warm to be healthy: 22-26 c. There’s a few nice wreaks and small reefs but no shore dives: most sites are a long boat ride out from shore. Best way to dive Brisbane is to get on a weekend dive boat (Big Cat Reality) that offers well priced two days diving out to the main sites. Mig-Diver's Pro Dive appears to be the biggest dive shop in town and from what I've seen and heard they are a good operation (though I haven't gotten around to diving with them yet...).
Cheers,
Rohan.