A hot water unit fluid supply control system comprises a switching device coupled to a hot water unit, a rainwater tank and a potable water source. The switching device is configured to selectively switch between a first state, which allows fluid communication between the hot water unit and the rainwater tank, and a second state, which allows fluid communication between the hot water unit and the potable water source. The hot water unit fluid supply control system comprises a control unit configured to: receive fluid characteristic information associated with fluid in the hot water unit from one or more sensors; activate or deactivate a pump of the rainwater tank based on a comparison of a fluid characteristic parameter of the fluid characteristic information with requirements of the hot water unit to thereby cause the switching device to assume the first or second state.