Hi Bryan,
Is it a one-way hire he's after ? If so then you're going to limited with the companies up here (I'm in Hedland), otherwise I'd guess you'll get slugged ~$800 to get it shipped it back to Perth. Those being Avis, Budget, Thrifty. There is a local mob in Hedland & Karratha, Mclarenhire but they don't have a Perth office. These guys do a lot of mine spec vehicles therefore they need to go offroad. Perhaps they may be needing to get a car up here. The number is 1800 227 444.
Another suggestion would be to look at Briz or Apollo hire. They don't have a depot here but could be worth a try.
Good luck