I am not familiar with Box Hill, other than the reference to it in the song, "Vincent Black Lightning!" But I agree that the PSPO is unenforceable due to its vagueness. OTOH, if local law enforcement elected to do so, they could arbitrarily "enforce" it to the point where it would be almost impossible to operate a motorcycle. Of course, that may be exactly what the locals wish to occur - no more motorcycles.
The same sort of thing has occurred in the US on well known "motorcycle roads." Typically they post low speed limits and install speed bumps.
Frankly, IMO the biggest cause of the issue is the internet. Some "influencer" writes/shows video on his website about some great road. Suddenly, the road is inundated with bikers. There are roads near us in the US that two-three years ago you could enjoy any time for the twisties/lack of traffic; now the same roads are slow, crowded and no fun at all to ride.