are some bases more in demand than others?
Absolutely! Right now there are vacancies in Sydney, Brisbane and Cairns. It's most likely that you'd be based at one of these initially.
Melbourne has had a waiting list for over six years. Time to get a Melbourne basing varies between a few months up to two years, depending on what you're after (i.e. FO/Capt/Dash-8/Q400). I can only think of one FO in the past six years who got based in Melbourne within three months of being checked to line.
Perth and Mildura have the required crew complements (Not 100% sure about Perth), but the wait for a basing there wouldn't be too long as many Perth crew are waiting for Melbourne positions.
AFAIK, all the Mildura FO's are waiting for Melbourne basings, so if you're after Mildura the wait may be short.
DIVOSH!