Mrs FR and myself have rented cars on our last three trips to AUS.
Each time we have come home with additional debts to the Australian government (plus two times when they have sent us photo-souvenirs afterwards from fixed radars). Twice we have been pulled over by Police cars flashing like a christmas tree with Starsky and Hutch style U-turns, for only being 5-10 km/h over the limit.
They are very strict and speed imits are often ridiculously unadapted to the circumstances, sometimes much too slow for no reason and then a few kilometres later you find yourself in a 110 km/h zone where the road conditions mean you would be suicidal to go anywhere near that speed.
HOWEVER, and I'm sure this will shock many contributors here and may not be condoned, as Europeans we have never, ever paid any of these fines and never had any problems as a result nor had our latest ETA's refused...
Just be reasonable and enjoy.