Here’s a quick guide to the laws around children’s car seats and booster seats in Australia.
When I did a check on some state laws, some did look different, for example on a Queensland Government website it says:
Children 7 years and over
Children who are 7 years and over may sit in a standard seat with an adult seatbelt, or an approved booster seat/cushion secured with an adult lap-sash seatbelt or an H-Harness. Or, they may be in an approved child restraint that is forward-facing with a built-in harness that is properly adjusted and fastened.
However, I definitely advise readers to only let their child to only use the adult lap-sash seatbelt when they are over 145cm.