Abstract: 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) a