Global water demand is expected to outstrip supply dramatically by 2030, making water recycling an important tool for future water security. A large combined grey water and rainwater recycling system has been monitored in response to an identified knowledge gap of the in-use performance of such systems. The water saving efficiency of the system was calculated at -8.5% in 2011 and -10% in 2012 compared to the predicted 36%. This was due to a lower quantity of grey water and rainwater being collected than the system filter required to backwash alone. Top-up potable water was therefore required to complete the backwash process and flush WCs and urinals. The efficiency of the filter itself was calculated at 0.68, much lower than current figures used in grey water saving predictions. Using direct mains water with no recycling system would be more efficient in this case, highlighting the importance of realistic system analysis before future installations.