Seems there is a lot of misinformation in this thread.
Referring to the WA legislation of the Road Traffic Act 1972:
<h2 class="title">Road Traffic (Vehicle Standards) Regulations 2002</h2>
62. Police inspection powers
(1) For the purpose of deciding whether to issue a compliance notice for a vehicle, a police officer may do any or all of the following —
(a) direct the driver to stop the vehicle;
(b) direct the driver to move the vehicle to the nearest convenient place to permit the police officer to examine the vehicle;
© direct that the vehicle is not to be driven, for a reasonable time, until the police officer makes a decision;
(d) examine the vehicle and any equipment or load carried on it;
(e) give reasonable directions to the driver about rearranging or removing, under the supervision of the police officer, all or part of the load (if any) carried on the vehicle so that the vehicle complies with these regulations and the Vehicle Standards;
(f) give any other reasonable directions to the driver about making minor alterations to the vehicle, its equipment or load under the supervision of the police officer so that the vehicle complies with these regulations and the Vehicle Standards;
(g) give reasonable directions about detaching a vehicle from a combination to permit the remainder of the combination to be driven.
(2) For the purpose of examining the vehicle or its equipment or load, a police officer may do any or all of the following —
(a) inspect and handle the vehicle, equipment or load;
(b) test, or require the driver to test, any equipment on the vehicle that is required to comply with these regulations or the Vehicle Standards;
© test drive the vehicle, or accompany the driver while it is being test driven;
(d) weigh the vehicle and any equipment and load carried by it;
(e) direct the driver to take the vehicle to the nearest place where it can be weighed;
(f) measure the vehicle and any equipment and load carried by the vehicle.
There is NO mention in the legislation where the police officer requires "reasonable suspicion".
It appears that the Police have broad powers to inspect your car; in your case, the Police seems to have followed procedure.
Also, the police may not enter your property without your consent. If they do so in order to sticker your car, it is trespass and you can lodge a civil claim in court.
Hope this helps.