[1] The Middle Harbour or Spit Syphon is a key component of the Northern Suburbs Ocean Outfall Sewer (NSOOS), which was the third major sewerage system to be built to service Sydney's rapidly growing wastewater needs.

The scheme involved replacement of all sewage treatment plants on the North Shore (with the exception of Hornsby) by a large main sewer with several branches.

Today, the system as a whole services areas as far west as Blacktown and bounded to the north by Narrabeen Lagoon, St Ives and Hornsby and to the south by Sydney Harbour, Lidcombe, Yagoona and Guildford.

The need to maintain a clear passage for navigation ruled out bridging, and the depth to solid sandstone beneath the Harbour bed made tunnelling impracticable.

It was therefore decided to carry the sewer across in two parallel pipelines laid on the bed of the Harbour where it was sufficiently deep not to interfere with shipping, and to operate it as an inverted syphon.

The twin pipelines encompass 20 reinforced concrete pipes, made in sections of 30.5m long and weighing 140 tonnes (150 short tons).

The syphon is a key component of the Northern Suburbs Ocean Outfall Sewer (NSOOS), the third major sewerage system to be built to service Sydney's growing wastewater needs.

The syphon remains a vital part of Sydney's sewerage system, it is still in first rate condition and has been in constant use since its completion.

[1] Middle Harbour Syphon was listed on the New South Wales State Heritage Register on 15 November 2002 having satisfied the following criteria.

[1] The place is important in demonstrating aesthetic characteristics and/or a high degree of creative or technical achievement in New South Wales.

The NSOOS, including the syphon, is a further example of the community's hidden heritage associated with public health and the quality of life.

The associated superstructures are of architectural significance displaying Art Deco style which is quite rare within the current Sydney Water system.