I don't think kangaroo has ever been the main source of the dingoes diet as they eat a variety of different animals from possum, to lizards, fish, birds and insects, they don't hunt in packs often and are pretty much lone scavengers that don't really go out of their way for a select prey. There are also two different types of dingoes that I know of, the alpine dingo and the desert dingo. Notice how the desert dingo is skinnier, it's because they have adapted to survive on very little due to living in an arid environment, so even if there was a larger population of them they are not actually designed to chomp done on massive amounts of meat very often. The alpine dingo eat more as it lives in a colder climate, but they mostly live in the south eastern mountain ranges, around Victoria's Alpine national parks where an over population of kangaroo the same problem compared to what it is further out west.