I would be extremely surprised if said rivers aren't already used for irrigation. And then there's the problem of overuse, leading to a river that is only a small creek by the time it reaches the ocean. (currently a problem for the Yellow River)I looked at a physical map, and in southeast Australia, it looks like there are some rivers. Just use those for increased irrigation and I bet you could sustain a lot of people there, potentially, as long as the focus remains centered on subsistence.
The question is why people would move there just to grow food.