JennieG, your son's experience reminded me of two vehicular encounters with wombats, one (coincidentally) at Wilson's Prom.

When I was doing my fieldwork at the Prom I was driving to Five Mile Beach in a Karman Ghia VW belonging to my supervisor and didn't see a wombat run across my bows. Even though I had been driving at about 30kph, the right front wheel assembly was pushed backwards about 2" and the wombat just ran off.

Some caving friends of mine ran into a wombat in their LandRover at Yarrangobilly. The wombat ended up wedged under the diff so they jacked the vehicle up to extricate the wombat, which immediately ran off. But the casing of their diff was cracked.

Score so far? Wombats; 3, cars; nil.
They really are the animal kingdom's version of a tank.

Cheers, Rowan