I haven't stayed at many hostels within Australia, but if you are in Sydney staying at a hostel around Kings Cross region is a sure way to find plenty of fun after dark (preferably somewhere up on the main stretch where most the businesses are, because down the bottom below the main business district it is a very dodgy and dangerous area after dark with very little light and lots of desperate, homeless people not to mention a lot of druggies). I know that I have seen one hostel down in that dodgy area I am talking about but I can't recall the name just the horrible yellow paint work it had.
I have found as a general rule around the world, the hostels that have dorms with a maximum of 6 to a room and usually same sex rooms with their own ensuite are less likely to be a party hostel. Normally the ones that have mixed dorms with 8,10,12 or more beds to a room and do not have an in room ensuite they are most likely to be a party hostel or at least have a lot of people who are party goers staying there. Normally the cheaper the hostel in the city, the younger the crowd it attracts so hence more likely to want to party, but this sometimes can also mean the hostel is a filthy place but normally not too bad.
Source: Travellers Point
Some great tips on party hostels in Australia below: