WA does not provide any relief if you only pass through an airport., so WA will treat them as coming from a low risk jurisdiction.
Refer to
COVID-19 coronavirus: Travel and border: State-wide for further information.
The current documents you need to see are:
Unofficial Consolidation of the Controlled Border for Western Australia Directions
Aircraft from Restricted Locations Arrival Directions
Approval for Flight Crew Pursuant to the Outbreak Outside of Western Australia Response Directions (No 5)
The directions change extremely regularly.
An aircraft can only disembark persons from a low risk location (ie the NT) at Perth Airport.
WA is extremely conservative and officials have history of imposing stricter requirements than contained in the directions.
There was another case of local transmission in the NT today, so I suspect the next time I'll disembark a passenger from the NT onto land in WA is February.