Firth Park is the best option
There are toilets and camping at Firth Park. It is 6.5km from firth park to the closest part of the wombat loop. The Loop is 16km (22 if you include the extra bits) so it could be a 35km round trip.If you want longer, there are heaps of fire roads and extra trails to explore.
I have made a google earth kml file if you want directions to the Wombat Loop from firth park. It can be converted into a gps file and followed if you have one.
Cheers,
Fergo