Probably because you are going so fast that due to the upsidedown nature of Australia, you car is trying to go into orbit. And the PS reservior is on the wrong side of the car.

I assume this is keeping steady pressure on the steering wheel while cornering. You need to check the level of the fluid with the engine off, of course. Bear in mind that the PS fluid is changing in those new cars and could be Pentosin based - check the owner's manual.

My first reaction is that the PS fluid is percolating because it is low. My second reaction is in regard to the belt slipping, seldom happens. Third is to look at the control arms for worn bushings.