My suggestion is to contact the Residential Tenances Authority.  They have 1300 number at their call centre.  The staff there will be able to tell you all you need to know about water usage and who pays.

As a landlord you have to provide a reasonable amount to the tenant.  If you want to charge for excess water then it has to metered and it has to be in th econtract.

However if a contract is in place now than you will not be able to make any alterations until it ends and can be renewed.

Ring Residential Tenancies Authority they can answer all your questions to do with rental properties.  It is not just for tenants it is for landlords also.