Meh, just spoke to the head of the Water Quality team at Yarra Valley Water, a guy called Brian Cole. He was very helpful but couldn't ultimately supply any exact quantities of the water components due to the mixing Spiesy mentioned above.
FWIW, the info referred to is in the link Yob posted in post #42 above.
Basically Brian was confirming Reservoir 3073 is supplied by the combination of Silvan + Sugarloaf + Yan Yean.
EXCEPT Yan Yean has been offline for the last few years due to poor quality of taste in the supply, and while water levels are good in the other reservoirs, will probably remain offline indefinitely.
ALSO, Sugarloaf = Winneke in the reports (Winneke is the treatment plant at Sugarloaf reservoir.
Finally, the supply to a particular location (e.g.: my house) on any given day varies between the 2 reservoirs quite a bit because the water supply system operated by Melbourne Water is quite dynamic. So depending on who's using a lot or a little water, the supply to that location could be coming mainly form one or the other reservoir, or a more even combo of both.
Basically it looks like i'll have to stump up for a decent pH meter.
The main ions vary a bit between Silvan & Sugarloaf/Winneke, but are generally all quite/very low in the grand scheme of things (as mentioned by manticle et al previously).
The main thing that seems to vary between the 2 supplies is the Alkalinity, which is obviously of major concern. Hence, i'll simply need to get a pH meter to be sure. Bugger it.
Any ideas for the cheapest good pH meter??