This catchment is defined as that area which drains to Middle Harbour and its tributaries. The mouth of the harbour is defined by Grotto Point at Clontarf and Middle Head.
The Middle Harbour catchment covers 100-square kilometres and has a population of approximately 200,000 people. Land use in the catchment is mainly residential, with some commercial and industrial use.
Bushland covers one-quarter of the catchment and includes Garigal National Park.
Councils within this area include: Mosman, Willoughby, North Sydney, Manly, Ku-ring-gai and Warringah.