As you mention Sydney Water I know your Plan is in NSW, so…
What’s actually happening is that the Owners of the 12 Units are each receiving a quarterly account from Sydney Water for “Service Availability” that, as the name implies, represents the cost of having its services available to each Unit to supply water and to take sewage away.
Sydney Water’s costs for all the water that’s actually consumed by those 12 Units is based on what’s recorded by that single water meter, and that quarterly account goes to the Owners Corporation who should be paying it from funds in the Administrative Fund and not as you say be “divided up evenly between the Units to pay”.
Payments for water consumption must, just like other common costs such as for building insurance, property maintenance (e.g. grass cutting), and electricity for the outside lighting be raised through Owners’ strata levies which are based on the unit entitlements applicable to each Unit (e.g. larger Units pay more), and as I said should be paid from your Plan’s Administrative Fund.
I too am aware that Sydney Water and other Water Supply Authorities have plans to meter the water supplied to individual Units in Strata Buildings in order to encourage each resident (as the water user) to use less and therefore pay less; that’s called “demand management“.
Whilst the technology required to do that has been available for years, many older Units have more than one supply inlet and so would need more than one meter, and so the additional costs to read all those meters combined with the potential for inaccurate metering caused by all the twists and turns within water supply pipes that weren’t designed for metering, puts the whole thing very much on the back-burner.
At the end of the day, the Water Authorities are paid for the Plan’s overall water consumption anyway, so don’t hold your breath!