The general speed limit tolerance is 10km/h for automated cameras. There is no official tolerance used by road police and they may prosecute for as little as 5km/h over the speed limit, depending on the circumstances.
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 may announce that the tolerance is reduced from 10km/h to 4km/h nationwide or in specific regions.
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.