Was in yamba last weekend. The locals reckon that 40kts has been seen behind the wall on a GPS.
Anyway, I tried to take a quick clip of some blasting behind the wall on monday, unfortunately its not very good cause i didn't close the camera in front of the air-con and it kept fogging every 5 minutes, plus sailing in a straight line for a long way ain't really a strong suite of mine. Its is probably 18kts behind the wall gusting easily to 20+ as my 5.9 was getting a overpowered in the gusts and i actually had a few sketchy moments despite the smooth water. Further towards the river mouth where u launch it was probably only 14kts, always seems to be about 5kts windier behind the wall. Tide was high so as you can see the waves spill over the wall in a few spots. But I think most of the bouncyness is because I didn't have the headstrap tightened up.
Anyway, enough talk, enjoy the vid. And just for reference on tuesday when I was overpowered on my 5.3 of pippi beach, the graphs were only showing just on 20kts, so I think the river there and the beach has a bit of a natural wind funnelling effect.