Has anyone checked for leaks?
Sounds like you’d be a prime candidate for water-saving meausres as outlined in this page on the Green Strata website.
Or maybe one of your neighbours has a nice little hydroponic cannabis growing venture up and running.
The best way to control water usage, especially in small blocks, is to check for leaks and then install separate water meters for each unit.
And if individual owners don’t want to pay for meters, you install them for those who do and then only charge them for the water they use, so that the others have to share the cost of all the rest of the water used. They’ll soon come on board.