A water supply system for supplying water to a building including a water tank (13) for collecting water from the roof (10) and gutters (12) of the building (11); a water level sensor means (21) for sensing the level of water in the water tank; a tank water system (22) for supplying water from the water tank (13); a water pump (23) for pumping water from the water tank (13) along the tank water system (22); a non-return valve (25) downstream of the pump in the tank water system; a pressure sensor means (26) for sensing water pressure of water within the tank water system, said pressure sensor means (26) is located downstream of the non-return valve (25); a mains water system for supplying mains water; a three port valve (35) having a first port (34) connected to the tank water system downstream from the pressure sensor means (26), a second port (37) connected to the mains water system and a third port (41) connected to the building's water reticulation system; wherein when the first (34) and third port (41) are open the second port (37) is closed and when the second (37) and third port (41) are open the first port (34) is closed; and a control means (27) for operatively receiving inputs from the water level sensor means (21), pressure sensor means (26) and mains electricity supply (45) and controlling the water pump (23) and the three port valve (35).