If you drive a car in Australia then you should know what to do if you hit an animal.
In particular, who to call if it's injured or may have young in its pouch. Around here it's Wildlife Victoria (03) 84007300.
You don't necessarily have to wait there, you only need to be able to give a decent description of where it is and maybe leave something as a marker by the roadside. If the animal won't be able to recover then someone, probably local police, will come and euthanise it.
This message brought to you by spending half an hour on the roadside this morning, after coming across a young kangaroo struggling in the middle of the road with a broken back. It's disturbing how many people hit animals and just keep on driving.