well as far as i'm aware, the GAP Australia program (
www.gapaustralia.org) is only for current school leavers. You apply while you're in year 12 and they do an interview.
You can choose to work voluntarily overseas in the following fields:
Caring - usually taking care of old people or kids.
Teaching - English as a Second Language
Boarding - Go and work in a boarding school, do some teaching aide stuff as well as the things that just need to get done.
Environmental - Help out with environmental and field studies, like those places kids go to on excursions for geography i'm guessing.
Outdoor Activities - Summer camp kind of stuff. Need to be experienced and have first aid qualifications and shit.
There are heaps of places you can go, China, Vietnam, Poland, the UK, New Zealand, Vanuatu, Russia, Germany, India etc etc etc.. the lsit goes on.
The whole thing (for like 6 months) will cost about $5000-$6000 (wild guess based on what I remember) and you can go travel for short holidays during the 6 months as well as however long you want after.
It looks like a good program and I intend to do it when I leave school next year