Not DH, but I started building/clearing singletrack and building step downs/steeps @ Redbank in Port Hedland whilst up there many years ago. May have been bulldozed since or access restricted? Basically, turn left when heading from South H to Port H near the Salt Farm and follow the road past the Landscaping Supplies place until you come to a large area of spoil (dredged from P.H. Harbour and dumped @ Redbank). Had some technical sections and about 2km of sweeping single track made by hand. Could get some decent speed happening through some sections and on some corners, plus some NS style skinnies etc
Only 3 of us used to build/ride there, and those guys have left the area too.
I hear ya, the road up to the water tower/lookout get very boring. I used to dig in the huge open drains and make some nice kickers to pass the time.
Like I said, may well have gone. Closer to home, contact the bike shop in Karratha, they may help. There used to be a MTB club up in Karratha