I'm not sure about SA, but in vic as far as I am aware, over taking another car doing below the speed limit does not require you to obey the speed limit.
for example, the car in front of you is doing 90 in a 100kph zone, you cannot premeditate the overtaking manure, but you can pull out and if its safe you can pass the car in front exceeding the posted limit.. IIRC, common law may say you're breaking the law on a technical basis, but you won't get booked for it..?
that is what I've been told by someone who has completed the police law exam studies.. whether or not they meant _they_ won't get booked, but the average Joe would, I'm not exactly sure
feel free to correct me..
edit: and if I ever come to SA, I would drive to the conditions of the road/weather.. most of the time is 20 over.. try drive from QLD to VIC or to SA on 130-140 most of the way, and slow for towns at 50kph, feel's like you're not moving at all.. :P