My understanding is that section 190(3) of The Criminal Code (being part of the Criminal Code Act 1913 (WA)) makes it illegal to be a "paid driver" for a prostitute.
This sections deals with "being involved with prostitution" and sub-section 3 specifically states: "Any person who lives wholly or partly on earnings that the person knows are the earnings of prostitution is guilty of a crime and is liable to imprisonment for 3 years."
You may also want to contact the Scarlet Alliance (the Australian Sex Workers Association) for further information in relation to your question.
Hope this helps. Please keep us updated with your progress.