Ok, the law states power output at the motor is supposed to be 200w max.
The rules state that there should be a label on the motor that says what the voltage and power output is. For instance my (recently sold) Oatley motor stated it has a max output of 250w at 24v. My Bafang motor has a label that says it is rated at 200w at 24v.
In actual fact, neither motor is technically running at those figures!
My newer hub motor has a label that says its rated at 200w (sic) at 36v.
For instance, the Oatley came std with a 15A controller, so thats 24 x 15 = 360 watt draw. At 80% eff, thats 288 watts at the motor! Increase that to 20A, and you have a motor capable of 384 watts, and thats a nominal 250 watt motor!
If you work out the same figures for my Bafang running at 36v and 15A controller, you'll see that it is putting out a healthy amount of assist!
But, it has a label that states that it is rated at 200w!
If I were checked by a cop, that would be the only thing he can refer to as to the power output of the motor, and as it says 200w, than I am 'legal'.
I would actually doubt that there are any systems out there that are 'legal'. I suspect that a lot of manufacturers/suppliers are quoting a nominal output as the max output, to circumvent the Oz law.
IMO, especially as a cop, and please, my opinion, NOT advice, is that if you have a reasonable motor output, ride/behave sensibly, then the chances of ever being convicted of using an overpowered e-bike are extremely small. Not impossible, but highly unlikely. E-bikes have the advantage of being very stealth, especially small hub motors! Most people wouldn't even know that there a motor on the bike!
If you don't give yourself a reason to be stopped by the local constabulary, then you are probably worrying for nothing.
I think the people who should be concerned, are those riding the ICE powered versions, as they are generally all overpowered, and bring attention to themselves due to the noise output. The other group are those riding the scooter type of bikes, as they are designed to be an electric scooter first, and a bike second! And they don't look like a push bike.
So keep it quiet, keep it stealth, obey the law, wear your helmet, make sure your motor is labeled, and you should be ok.
If you really, really want to stick to a legal 200w, then thats up to you. I would be looking for a BB drive for that kind of power output, and quite probably PAS. You couldn't rely on that sort of power to give your legs a rest, and coast on the motor for a bit.