If you're holding your formal in the city, the Blackbird Cafe at Cockle Bay Wharf, Darling Harbour holds them. A guy had his birthday on the day of our formal, so he went to the trouble of organising an official one for everyone at the Blackbird for a mere $25 p.p, drinks not included (and they are very pricey there) but there was a continuous supply of food. Given the dearth of places that let both over and under 18's in, it was a great venue, those of legal age were simply given wristbands (and those underage were easily catered for). We had our formal at the Four Seasons and the dj there sucked, but the Blackbird was great, chalk and cheese. Party lasted til 3am, though by then only about 10-20 were left, most people lied about staying til the end and left earlier.
We didn't really book our party until this time of the year, so it's not too late. Cruises are an alternative, but the catch is you can't get off it if it sucks, better to stick to land. I don't know of many other venues that can host afterparties for all ages except the Cockle Bay wharf (Wallaby Bar does them too I think), but otherwise look around for licensed cafes, they're your best bet. An 'official' formal party is really only good if your year is tightknit and the majority will go, otherwise people will just have to organise hotel rooms and the like, which are a lot more expensive.