Yes, Colin, there is a lot of roadkill in rural areas, particularly kangaroos and wallabies. Collision with a roo, especially a big one, can cause considerable damage to a car, and is usually fatal to the roo. On our trip earlier this year, while travelling between Cunnamulla, near the Queensland / NSW border, and Bourke in western NSW, we saw the remains of dead roos on an average every 100 metres. Most of the remains were little more than desiccated skin and bones, and were quite old, but there was still a number of fresh kills from the previous night - most collisions with wildlife occur at night, with dawn and dusk being the worst times. Most of the deaths of roos etc on country roads are caused by heavy vehicles such as semi-trailers, B-doubles and road trains. These vehicles don't stop quickly and, as they are fitted with bull bars, just keep going, to attempt to avoid a roo could cause an accident. In some areas, emus are a problem, and they, too, can cause a lot of damage to vehicles.
I never travel in roo country at night in a car, it's not worth the risk, but during my coach driving days it was often necessary. I did hit a number of roos, they can suddenly decide to cross the road just as you approach, and there's no time to stop. BTW, it would be almost unheard of to hit a platypus, as they are aquatic, very shy, and rarely seen.
Peter.