"carriageway" means a portion of a road that is improved, designed or ordinarily used for vehicular traffic, and includes the shoulders, and areas, including embayments, at the side or centre of the carriageway, used for the stopping or parking of vehicles [...]
In conclusion: in WA, it's illegal to stand on the shoulder of the highway; however you may hitchhike from a footpath or off the shoulder. A law enforcement officer might not be aware of this, though. The source is to be found in [http://www.austlii.edu.au/cgi-bin/sinodisp/au/legis/wa/consol_reg/rtc2000113/s259.html?query=hitch%20hike thiswebsite] website.
=== Practical Situation ===