Melbourne Water will supply the retailers from where ever they have water to provide, the retailers (CWW, YVW, SEW) will do the same from their own supply provided by MW.
Yarra Valley Water will be the hardest to determine supply because they can receive from anywhere, the others, particularly CWW cannot. Most of Yarra Valley Water's water will come from catchment dams of Melbourne Waters, into Melbourne Water's Sugarloaf dam and through the Winneke Water Treatment Plant at Sugarloaf then into the Melbourne Water distribution network (including other large tanks and storages not listed on their site as they are not catchments, eg. Frankston Res, Mitcham Res, etc), then into the YVW distribution network including their tanks, then into the reticulation network.
As for the water quality districts with YVW (
http://www.custdetail.yvw.com.au/waterquality/default.asp?style=business) they are not storage specific, they are just where distribution goes into a reticulation network.
What is the most complicating factor in all of this is now we have the desal, they seasonly change the direction of the water flow in Melbourne in the distribution mains. This does change quality and water make up significantly.
All the retailers can and will change supply lines daily in some cases. They might receive water from Melbourne Water for a week or two, top up there own storages, then distribute as demand requires. This would mean the same reservoir water is split up, put through different systems, but potentially go to the same end location.
When you have a look at large water users or food/beverage producers, like Amcor, CUB, SPC, etc (the one's I'm farmiliar with), they all have inhouse water monitoring and treatment. Because varaition does occur at any point in time.
You can read what ever report you want, but its a yearly, maybe monthly average at best.
Your only a homebrewer, so accept there are variables you can't control to the nth degree.
In summary:
Melbourne Water will not know what water got to your house. They can't even accuratley speculate.
The retailers (YVW, CWW, SEW), don't really know what water Melbourne Water supplied them. But they might be able to tell you what tank was supplying your area at a particular time. Big might, but it won't mean much.