Hi ozmacca,
The Mill is located in Georgetown (90km north of Agate Creek).
The Robertson river does not run through Georgetown.
http://www.bonzle.com/c/a?a=p&p=209828&cmd=sp
It's irrelevant, but the Robertson indeed doesn't "run" all year round, but it's rarely "desert-dry".
Not sure of the Mill's water source, but I'm guessing the same as the town's - Etheridge river, bores and dams.
The Mill has been around along time.
Cheers,