You can do an initial free consult with most automotive engineers.
I went to Tonkins @ Wayville and discussed my original concept prior and also applying through Transport SA.
To be honest applying to the authorities is useless as all it states is that they agree in principle - you will need engineering approval etc. You also only have a number of years (4 years?) to complete the project for inspection or you'll need to re-apply. Mine ran out a LOOOONG time ago!
Enquire with an engineer first as they're the ones that Transport SA will need to sign off the mods regardless. They just basically register the thing with the mods.