Kieran,
the registration and compliance is quite a simple process to go through, there are a few guys around that can do it for you.
The road worthy is noting special as any old mechanic can do it.
The compliance required is an L02 compliace, which is a safty compliance. All that is required is the installation of new australian standard seat belts, side intrusion bars and alterations to lights. this is all.
if you are handy you can do it all yourself and get an engineer to sign it off, or you can do it the easy way as most people do. Get the engineer that signs it off to do the work.
As far as im aware you are requied to install 3 point retractable seatbelts.
if your after a list of the engineers that can approve the modification I would suggest ringing queensland transport and requesting the list. remember only ask for the list that does Lo2 otherwise you will get a list of every regesterd motor mechanic around that does compliancing, and beleve me tha is a big list.
If you get one of these guys to do your mods as well as the compliancing (mod plate) they should charge anything from $800 to $2500. There are cheaper guys around, but as usual you only get what you pay for.
Before you start any of this make sure you have all engine and vin numbers, if you don't have any one of these book your car into see a senior detective with cib police, they will need to inspect the car and authorise the issuing of a surrogate vin. this process can be quite time consuming.
we there it is I hope your registration process is easier than mine was
cheers
Adam
Good luck and happy mustanging