Do the unit owners with gardens keep the gardens in good nick? Do they enhance the appearance of the building? Perhaps the common burden of the water bill should be seen in the light of maintaining the common amenity.
Are you absolutely sure the gardens belong to the garden units? Are the garden areas on those unit titles? Or might they actually be common property that over the years has been fenced and sort of made a part of the garden unit?
Just out of curiousity, are you in Sydney? Why on earth do you need to put drinking water on your gardens? Doesn’t it fall from the sky pretty regularly?
Information here on how to change (add) a by law:
You’ll need 75% of owners. It seems that numerically the apartment dwellers outnumber the garden units, so you may get that. Unless the garden units are large in comparison to the other units and they may add up to a greater “unit entitlement”.
This document tells you a lot about installing meters:
You might like to get a quote from a plumber about how much this will cost; that may put the complainers back in their box.
I agree, the garden unit dwellers may have something to say. After all, some may be water misers with water-wise native gardens; their water usage may pale in comparison to the guy in the apartment with a hygiene fetish who showers 4 times a day!