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!