A first flush rainwater diverter and collection system for use with rainwater storage tanks primarily in domestic situations. The system comprises primary (13) and secondary (14) storage chambers, a T-piece (10) for connection in a rainwater flow path, a fall pipe (12) connected to the T-piece and extending into the primary storage chamber, a float within the fall pipe which seals on a seat when the level of rainwater reaches a preset level, a rainwater flow regulator (17) which regulates the flow of rainwater from the primary storage chamber to the secondary storage chamber, an overflow (18) and an outlet (19) in the secondary storage chamber.