People are blaming corruption but it's more difficult than that and underpinning it (I think) is perhaps a bigger issue.

In addition to farmers and residents owning varying water rights depending on what they're doing and the size of their property some of those people also sell the water rights of their property (or part of them) to another property. A hobby farmer doesn't need as much water as an industrial-scale rice plantation so the rice farm will purchase the part of the water rights from the hobby farmer. Additionally, that hobby farmer might scale down later because he/she is getting old and sell off some more of their water rights to a different company. Maybe the rice farm needs to downscale sells some of the rights they purchased, plus some of their original rights to a third party. Add in companies/people who buy land for no other reason than the water rights and stuff turns into a right mess real quick.

I'm sure there's something to be said about such a thing being more regulated but it's kind of difficult to track and the selling of water rights does have some merit to it beyond the stereotypical retiree or foreign, industrial farming operation killing the environment. Solar farms and other non-agricultural businesses quite often sell off part of their water rights to help pay for the original setup of the operation. Crops like hazelnuts and dates which require very little water do the same - hell dates are grown near Alice Springs where water rights fetch a hell of a penny so it's an attractive prospect for an industry we really haven't tapped into here in Australia properly.

In matters drenched in politics, incompetence and mismanagement it's always tempting to just throw up your hands and say that it must just be corruption or "technically not corruption" or personal interests. The reality is that it's almost always a bit more complicated, and quite a bit more interesting, than that. Reality resists simplicity.