A water supply system that supplies water to a plumbing system from either a mains supply, or an auxiliary supply. In a default state, a shuttle valve is pressure balanced and couples water from the mains supply to the plumbing system via mains inlet port and outlet port, and in another state the shuttle valve is pressure unbalanced and moved so that auxiliary water is coupled to the plumbing system via an auxiliary inlet port and the outlet port of the shuttle valve. Water is drawn from an auxiliary tank and coupled to the plumbing system when two conditions exist, namely when water is demanded, as sensed by flow sensor, and when water is available from the auxiliary supply, as sensed by float system. Water pressure at the auxiliary inlet port causes the shuttle to move from the default state and couple water from the auxiliary inlet port to the outlet port and thus to the plumbing system. When either of the two conditions are not met, the shuttle valve remains in the default state and couples mains water from the mains inlet port to the outlet port and thus to the plumbing system.