I've posted on this previously eons ago, and it's not what you think it is, and you have to go back to the original legislation circa 1870's to understand its original intention and recognise how it's since been skewed in our times for another purpose.
As I understand it, in the Colony of Victoria the provisions of original "Miners Right" allowed the holder to peg a "Miners Right" CLAIM, and to occupy that claim, meaning that he could erect a tent or throw up a humpy and live and work on his claim. This lead over time to a hapzard string of shanty type settlements springing up alongside watercourses. The colony government seeking to encourage settlement in surveyed and gazetted townships enabled legislation "exempting" certain rivers and streams from occupation to enforce centralised settlement. You will see on many old town and parish maps in Victoria on land within the town boundary that is adjacent to watercourses for so many yards, chains, or links on either side, that it is designated as "reserved for Public Purposes". Fast forward 160 years or so and the original legislation has become a useful instrument for the control of water quality, land management, fisheries, and forest management purposes etc.
And yes! as had been said by other posters, the exempted rivers are the ones with gold, and now you know why.
And another truism - if you are prospecting and you can't see the roofs of the nearby town then you are not on the gold!
casper