Did 12000' AGL in a 12 knot (1200 feet/minute) thermal at Beverley (Western Australia) a few years back. In a twin Acro.

Pingpong ball on a fountain! Needed to use serious angles of bank to avoid being spat out of it.

Cloudbase was still at least 3000' above us at 12 grand and it was still 12 up but the guy in the back (quite rightly) started muttering about oxygen.

So we knocked off a 150 km triangle. Hour and a half from launch to landing.

Wave in Scotland is fun (my best ever was 21000') but it's not like Oz!

So I'm assuming "Greenbank"... if you put "Greenbank Queensland" in the text entry box located at:
Gliding Australia
You'll see all the GFA clubs and approx. distances from your son's place.

Unfortunately, most club based gliding clubs in Queensland only fly on weekends or public holidays. To get clubs running 7days you have to travel to NSW or VIC.

Closest seems to be Boonah Gliding Club (46km away) - which at one point use to fly during the week via prior appointment in a K21 self-launcher.
G.Dale coaches at Kingaroy Soaring Club (170km away) from time to time...
I did most of my flying at Darling Downs Soaring Club (151km away) - as it was the closest to my parents place.
Lake Keepit Soaring Club (8hrs drive away) is now managed by an ex-Lasham staff member and from all accounts comes highly recommended if you want to make a week of it.

Another of my Kent Gliding Club instructors from the '70s was associated with this Australian gliding club I understand - John Hoye.

Brilliant! I know Boonah, not far from where son lives. Didn't even know there was a club there. Pity about the weekend only bit, I don't want to be disappearing off gliding when he only gets weekends off.