I've spent considerable time on the beaches all the way to the far north, and they are pretty clean. Strikingly so by European standards. If something evidently from an aircraft washed up on a beach there'd be a good chance of spotting it from the air, I reckon. First BUT: from Kalbarri to Shark Bay is largely tall cliffs, not beach. Second BUT: the ocean currents don't appear generally to move stuff to the WA coast from out there, so the whole question is probably moot.
There is a lot of holidaying that happens along the coast, people 4wding up the beaches, camping there for weeks at a time, etc. It would be worthwhile, I suggest, releasing official images of the most likely buoyant items that might wash up somewhere, whether that's Christmas Island or Madagascar – e.g. seat cushions, life jackets, escape slides, whatever. How many members of the public would even know what colour these items would be on an MAS 777?