The general speed limit tolerance has been 10km/h for automated cameras, however, according to the police, this has been reduced and they are remaining tight-lipped about what the actual tolerance is. There is no official tolerance used by road police and they have the power to prosecute for as little as 1km/h over the speed limit, so you should aim to keep below the speed limit.
Areas around school zones have a lower tolerance of 4km/h. This is because school zones are signposted at 40km/h and a 10km/h tolerance would simply allow drivers to maintain a speed almost that of the urban national road limit.
During holiday periods police have previously announced that the tolerance is reduced from 10km/h to 4km/h nationwide or in specific regions, but for all we know, the tolerance might now permanently be somewhere around 4-5km/h.
Vehicle speedometers are deliberately inaccurate, overreading by several percent. It’s not uncommon for a car that reads 55km/h on the speedo to be doing 50-51km/h. Therefore that cannot be used as an excuse for speeding.
There are many GPS apps for your phone which will give you a readout of your speed accurate to around 1km/h.
Why is having a speed limit tolerance common sense?
The speed limit tolerance is a common sense acknowledgement that making drivers nervously check their speedo all the time takes their eyes away from the road. Road safety advocates might say that if you can’t maintain a constant speed then you shouldn’t be driving, but that ignores the reality that many drivers (who are otherwise safe drivers) do find it difficult to maintain a constant speed and might fluctuate +/- 5km/h around their desired speed for a number of reasons, such as the road’s gradient, or inattention. Even many cruise control systems are variable within a few km/h because they are reactive to changes in gradient, not predictive of changes in gradient.
Even vehicles with speed limiters, e.g. trucks limited to 90km/h, can exceed the speed limiter’s speed.