Middle Harbour, a semi–mature tide dominated drowned valley estuary, is the northern arm of Port Jackson, an inlet of the Tasman Sea located north of Sydney central business district on the coast of New South Wales, Australia.

Port Jackson, consisting of the waters of Sydney Harbour, Middle Harbour, North Harbour and the Lane Cove and Parramatta Rivers, is the ria or natural harbour of Sydney, New South Wales, Australia. The harbour is an inlet of the Tasman Sea. It is the location of the Sydney Opera House and Sydney Harbour Bridge. The location of the first European settlement and colony on the Australian mainland, Port Jackson has continued to play a key role in the history and development of Sydney.