I can't see why there would be any special regulations for the spacecab - as long as the seat fits and the seatbelt holds it correctly.

If you are worried, you can call your local QLD Ambulance (not sure if they do it in mt Isa but I can't see why not) and ask them to check that it is correctly installed - they do this for free. We had to make an appt at our QLD ambulance for ours but some of them you can just drop in.

Legally, Australia' child restraint laws are actually pretty bad. You have to have your child in a seat that passes the relevant Australian Standards (they should have a sticker on them that says this) until they are just 12 months old and that is it. I think they are reviewing this at the moment.