its against the law to face your child backwards after 12 months of age in Australia, regardless of weight.

Its not just a fine from an officer you need to worry about its a charge for child endangerment which could see you go to gaol ... Just use a Seat that meets Australian standards.... your ds will be able to be in a rear facing one for a whole year yet....
That is not true the only law is not to turn before 8kg or 6 months minimum, and most rear facing car seats have a max weight of 12kg. DS1 was only turned at 17 months as neede car seat for DS2 and DS2 still rear facing at 15 months.