is it allowed?
Organised tours operate but there's nothing about going out alone....
www.parks.vic.gov.au/places-to-see/parks/wilsons-promontory-national-park
I'm pretty sure that most tracks cannot be used for mountain bikes. However the track from Darby saddle car park to the lighthouse was allowed. It is a 23 km ride to the lighthouse then 23 ks back to the carpark. I'm not sure the upper carpark is open due to damage on the road b/c of the floods down there a while back.