Bird with a loud call/screech at dawn

by Peter Huepler
(Germany)

Hi - it's been a few years, so memory is hazy.

I'm trying to identify a bird I met during one of my stays in Australia. I was somewhere between Sydney and Newcastle, along the coast. We we staying at a motel by a lake (apparantly an artificial lake, made when mining was no longer profitable or there was no more coal to be found.)
Next to our cabin was a broad sprawling tree, which at 5am was brimming with big white birds with a crest on their heads - I seem to think something in the line of cockatoo/cockatiel/kakadoo... not sure.
Either way, as the sun came up we nearly fell out of our beds as they suddenly all started up with this 'Whuaaaaarrrrrk Gak gak gak gak gak gak gak' sort of call - can't describe it better. But it was incredibly loud - multiplied by the numbers found in the tree.

If anyone has an idea of what this might have been, I'd be grateful.