For cars manufacturerd prior to July 2006, ADRs were + or - 10% accuracy on speedometers at speeds above 40kph. However, after July 2006, the ADR was changed - it's pretty complicated, but you can read it all at the link below:
http://rvcs-prodweb.dot.gov.au/files/ADR%201803.pdf
Essentially under the new ADRs, you cannot have under-reading (as described above), only over-reading - so yeah, it'll always display a higher speed than you're actually doing.
Either way, the new ADR does not apply to most of the car owners on SAU as our cars were manufactured prior to 2006.