Landlords, letting agents or Local Authorities are required to inform Welsh Water about tenants in their properties within 21 days of the tenants moving into the property.
If this is not done, the landlord can become jointly and severally liable with the tenant for any outstanding water and sewage charges.
Landlords will need to supply details to Welsh Water, which will include the property address, tenancy start date and title, name and date of birth of all adult residents.
They can apply online and landlords can register details on a dedicated website www.landlordtap.co.uk
The full details can be found on the Dŵr Cymru website.