We went to the online RSPCA knowledgebase to look for advice. While they don't have a specific policy on the appropriate restraint of dogs in cars, they do have a policy regarding containers for transport.
The RSPCA says "The container should enable the animal to lie down comfortably in a natural position, turn around, stand and sit erect and stretch with clearance. Another method, for drivers with station wagons, is to put the dog behind a cargo barrier." The RSPCA can also issue fines under The Prevention of Cruelty to Animals Act. If an animal is injured because it was unrestrained, owners face up to six months’ jail and fines of up to $5,500. Carrying dogs untethered on the backs of utes can land drivers with fines of $500.
Read more: What are the laws around transporting Pets in cars?