The legal speed limit for electric bikes in Australia is 25km/h. This is the maximum speed allowed for an electric bike with motor assistance. The motor power must cut out and stop providing motor assistance when the speed reaches above 25km/h. Off course, you can pedal fast and achieve more speed with the electric bike.
Any electric bike that goes faster than 25km/h must be registered as a motor vehicle and the rider must obey the same road rules as motor vehicle drivers.