"There is not a lot anywhere North of Perth, hundreds and hundreds of kilometres of mostly deserted coastline."
You're joking right?
The Carley float from HMAS Sydney in 1941 ended up drifting to Christmas Island (found 3 months later after sinking off Carnarvon). The Indian Ocean currents tend to circulate away from the WA coast in the Exmouth area. Although it may depend a bit on the wind direction as well.
IMO more likely to wash up in either Indo, Christmas Island, Maldives etc .
