Practically an electric bike can go as fast as you are able to pedal it. However, as per government regulation in Australia, the motor power of an e-bike/scooter must cut out (stop providing electric-assist) automatically when reaching the speed of 25km/hour.
If an e-bike is not legally compliant it must not be ridden on roads and paths. They may only be ridden on private property that isn't accessible to the general public.