Speed camera's are outof hand now in sydney, page-9

"All cars fell within the parameters of an Australian Design Rule that requires that the speedo must not indicate a speed less than the vehicle’s true speed or a speed greater than the vehicle’s true speed by an amount of more than 10 per cent plus 4km/h."

Read more: http://www.drive.com.au/motor-news/how-accurate-is-your-speedo-20140716-ztmh4.html#ixzz46hOSBcyR
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Sometimes incurring a fine is not voluntary. This issue is handled in the UK and most of Europe with plain old common sense. It's based on the flow of traffic. Point to point cameras determine the speed of the majority of traffic. If the speed of the traffic flow is above the limit no fines are incurred. If we have a few clowns exceeding the average speed of the flow, they are fined. Very simple and very British.

Australia, on the other hand, seems determined to do it their way regardless of the evidence and common sense. So, it can only be professional stubbornness or revenue raising, or both.