Yep welcome to WA I say.
About the mod plate \ permits...
Here is the relevant section on Transport website: http://www.transport.wa.gov.au/licensing/20500.asp
Essentially most things will "require approval" to be modified if you want to be squeaky clean.
This involves filling out one of these http://www.transport.wa.gov.au/mediaFiles/LBU_F_VS_ModificationLightVehicles.pdf
waiting about 4-8 weeks, then probably requiring engineering approval - contracting a consulting engineer to write a report which gets submitted to WA transport then if approved finally pit inspection to gain permits to carry in your car.
Some things...like coilovers, turbo conversions etc you will get away with (most times) on the road and not many people bother to get them engineered.
For example, I took a NA car over the pits with a turbo conversion that was well done and they passed the car without engineering.
But technically, if I get pulled over and the cop is having a bad day, I can get done for no permits for the turbo.
Usually, engineering is used for engine swaps (ie rb20 to 25/26) or cages etc.
I think thats a rough overview covering most stuff for you.
Anyone else feel free to correct me if something isnt right