Presumably you are talking about water consumption charges rather than the fixed or standing supply charge billed directly to the owner. If you are in NSW, and only have one meter for the complex, then the account for water consumption would have been sent to the Owners Corporation for payment. As such you should be paying for consumption as part of your levy charges from the Admin Fund, and your levies should have been assessed according to your unit entitlement. Unless all your unit entitlements are equal you should not paying an equal amount for consumption to the other owners. If some units have more land than others within their lots, then presumably their unit entitlements would be greater and they would be paying more through their higher levies.
Do you have a Managing Agent? If so he/she should be able to explain it to you.