Hmm. Usually GST is applied to levies. The OC pays for stuff that adds up approximately equal to the levies. The OC claims the GST back on all that stuff. GST in and out end up cancelling out.
I could be wrong but I suspect your OC will have more GST in than out and would end up having to pass on the excess GST. Another good argument for metering water for each unit and each unit being billed separately by the water supplier independently from the OC.