The VicRoads page on scooters and wheeled recreational devices states:
If your motorised scooter:
- is powered by a petrol motor
- has an electric motor with a maximum power greater than 200 watts
- has a maximum speed greater than 10 km/h
then it cannot be legally used on a road or any road related areas, including footpaths, share paths and public areas. The fine for an illegal device is $826. Other penalties may also apply.
All of the most popular models (eg. the Segway Ninebot E25 and Xiaomi Mi 1S) are capable of going beyond that speed.