Water tank - tenant or landlord responsibility?

Hi,

We have a water tank that uses rainwater first. If there is not enough rainwater, it switches over to mains water. There has been an alarm sounding on it for the past couple of days and it has switched over to mains water.

We are in a rented house and will need to call out a technician. Is it the tenant's responsibility or the landlord's to get this fixed?

Thanks!