Queensland options...
At a guess I'd recommend Cairns. GBR diving (either day dives or liveaboards), land based things to see and it's a party town that caters to young tourists, i.e. drinking, restaurants and Fun. It doesn't have a beach, but if you have a car there are beaches within an hour or so's drive.
Top things to do in Cairns - Lonely Planet
Another option would be Byron Bay in NSW. It has more of a laid back indie vibe, great diving off Julian Rocks and also caters to young tourists with the aforementioned drinking, restaurants and parties.
http://www.sundive.com.au/
Likewise Airlie Beach in NQ. Though I haven't personally dived there, it has a good local dive operator and is a well-known party town with a beach and lots of foreign backpackers.
http://whitsundaydivecentre.com/scuba-diving-whitsunday-islands/
The Gold Coast is Queensland/NSW's Schoolies destination of choice (the Australian equivalent of Spring Break); however the diving is just reasonable. You can have a nice dive there, but IMO Byron is a lot better. However, it's a much bigger city and has a lot of land based entertainment (Seaworld, the casino, lots of nightclubs, chicks in bikinis and fun in the sun) if that's what you're after.
If you're primarily coming to go diving I'd go to Cairns/Port Douglas, dive the GBR and party at night. If diving's a big factor but not the prime motivator, I'd think about Byron or Airlie. If partying is the name of the game and you'd like to get a dive in between cocktails, I'd go for the Gold Coast.
Of course there's also Western Australia, but I don't know all that much about what's on offer.