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.

Seaforth is a suburb of Northern Sydney, in the state of New South Wales, Australia. Seaforth is located 12 kilometres north-east of the Sydney central business district in the local government area of Northern Beaches Council and is part of the Northern Beaches region.