As a Melbournian who mainly attends prog/power/thrash gigs I can say that we don't have many great venues for the smaller/ underground scene. I think the "top" three (very lose term) would be the Arthouse (rough capacity about 150- 200), Musicland (~250) and the Central Club (~350). Of the slightly larger venues which can work for a larger local lineup or underground international gigs there's the Esplanade hotel/ Gershwin room (~450), The Corner hotel (~600) and the HiFi Bar (~700-800).
They all vary in their scumyness (for want of a better word) and the sound/lighting equipment ranges from half decent to more than capable, depending on the band and techs.
There don't seem to be any "regular" bands who keep up appearances all year round as they go into writing and recording stages, or life gets in the way (im sure you know that story). This year in Melbounre Eyefear would have to have had the most amount of gigs for a larger prog/power act, with some decent International supports.
Lately I've been able to attend gigs almost every weekend and the numbers have ranged from the usual 100-150 (eg, last nights Mortal Sin show at the Central club) to 300+ for the Vanishing Point album launch at the HiFi the other week.
I think our scene is quite a decent size, but the problem is that it's always the same people that go to gigs. The Vanishing Point gig was a great exapmle of where our scene needs to go, becuase I didn't recognise over half the people there from gigs of previous weeks as I usually would.