Decentralised hybrid rainwater-greywater systems can switch between climate-independent greywater during dry seasons or climate-dependent rainwater during monsoon seasons, resulting in higher water savings than either rainwater harvesting or greywater recycling systems alone. However, hybrid systems are not widely adopted due to a lack of data on untreated rainwater and greywater quality and a paucity of case studies on pilot-scale systems. This study aimed to monitor untreated greywater quality from two full-scale greywater recycling systems and to assess the performance of a decentralised hybrid rainwater-greywater treatment system operating on a pilot-scale under controlled conditions. Both mixed greywaters sourced from washbasins and ablution activities, and from washbasins, showers, baths, and laundry discharges must be treated prior to reuse: untreated greywater was frequently contaminated with faeces (20/32 samples tested positive for Escherichia coli), and exceeded the allowable Malaysian limits for both recreational waters with body contact (Class IIB) and irrigation waters (Class IV) for total coliforms (27/32), biochemical oxygen demand (BOD5) (19/32), chemical oxygen demand (COD) (12/32), colour (8/32), turbidity (24/32), ammonia (NH3-N) (16/32), phosphates (PO4-P) (28/32), and manganese (Mn) (4/14). Principal component analysis yielded 4 principal components: organic matter from food and body residues; detergents; faecal contamination; and ammonium salts. The pilot-scale hybrid treatment system featured a multimedia filter (MMF), a granular activated carbon filter (GAC), and ozone disinfection. A hydraulic loading rate of 10 L/min produced the highest overall removal efficiencies as longer retention times allowed more pollutant adsorption. Overall, the pilot-scale system removed 52% COD, 53% BOD5, 14% NH3-N, 67% PO4-P, 81% colour, 81% turbidity, 50% total suspended solids (TSS), 53% of total coliforms, 63% copper (Cu), and 29% zinc (Zn) from greywater sourced from a mixture of showers/baths and laundry. The GAC was the most effective at removing COD, colour, turbidity, and Zn. Finally, a series of dilution experiments of greywater with rainwater and mains water was conducted to emulate real scenarios during the practical implementation of hybrid systems with mains water top-up. Greywater dilution had little impact on removal efficiency of colour, turbidity, TSS, and TDS, and dilution of greywater with either mains water or rainwater is recommended to ensure that pH, BOD5, COD, NH3-N, and PO4-P are within Class IIB limits.