Hi Frostfire
Here there is a commercial kangaroo industry that "harvests" wild kangaroos (mainly for meat and skins). Guess what the total kangaroo quota was for 2007 - have a look at the links below - you may be surprised. There are over 50 species of kangaroos and wallabies, but only a handful of them are killed commercially. Crop damage and competition with livestock are the main reasons the farmers want the numbers reduced. Now when you get large populations in areas closer to the cities (like there has been in Canberra), bringing in the shooters would not go down so well, so other solutions like contraceptives are being tried.
The story is different with Koalas. They are declining in certain areas, and need protection. However there are some localised areas like Kangaroo Island where the population has been growing to the point where there are not enough trees to feed them.
Here are a couple of links that may be of interest.
Kangaroo harvesting - EPA/QPWS
http://www.environment.gov.au/biodiv...est-quotas.pdf