If I was in your situation I do my utmost to have as little to do with the seemingly incompetent developer/owner/manager as possible, and that includes avoiding any common water bills, whether they be mains bills or tank bills. It may be several years before all 14 lots are sold and it would appear that your life will be continually beset with the problems that you have already mentioned.
If it means paying an extra couple of hundred dollars to install another water meter for the tank then that is the option that I would chose. In other words, I would gladly pay for the two separate meters. Make sure that the water meters are easily accessed by the meter readers.
Trying to read and calculate the correct amount that you have to pay from the mains water meter for the common property is not something that I would like to undertake and may not be necessary, particularly once an OC has been established.
“The individual meters for the units are located downstream of the master meter and their consumption is deducted from the master consumption for billing purposes. Any residue (common area usage) is billed to the Owners Corporation.”
The Owners Corp common property bill for common property usage is paid by the OC from the Admin Funds. Unlike water for personal use, the common property water usage should not fluctuate too greatly so should be able to be ‘guesstimated’ and assessed before an AGM and included in the levies. A plumber may be able to assist you set the first common water Admin Fund line item for common water.
I am not sure what region you are in but the Hunter Region have produced a booklet that may assist your scheme in installing separate meters. Perhaps you have seen it? It includes a lot of helpful advice such as best practice for installation, by-laws etc.