Hi Brent,
On the racing scene we have VMXWA (pre75) in Narrogin 190kms from Perth, Classic MXWA (pre 95) in Darkan same area roughly and Vinduro WA holds a few events around the southwest, Narrogin and Darkan are clay/gravel surfaces and fun tracks to ride. We also hold a few Natural Terrains through the year, check out the relevant clubs.
Plenty of Legal trail riding for registered enduro/trail bikes around the Harvey/southwest area and a couple of places north of the river if you have offroad rego. Depends on where you ride but a good mix of sand and lots of hard pack and gravel 50/50 of our mx tracks are hard pack or sand.
We have 3 offroad ride parks all within 2hrs of Perth and there are a few organized trail rides like the Adventure Rally etc, hope this help with you decision.
Cheers
Micko