Hyde Park Obelisk

There are two types of sewer vents within the Sydney Water system: educt and induct. Induct vents draw air into a sewerage system to aerate the pipelines. Educt vents allow gases to escape when the gas is lighter than air.

The shape of the cowling on the older types of vents were set to produce either eduction or induction whenever there is a natural breeze. This is an educt shaft, a simple masonry shaft modelled on Cleopatra's Needle (now situated on the Thames Embankment, London) and erected in 1857 during the period of George Thornton as Mayor of Sydney.

Vents are for the safety of personnel as well as to prevent corrosion by chemical decay. The ventilation of sewers is a very important facet of continued operation of a sewerage system; poor ventilation can result in serious odour problems and material corrosion.

Unveiled in 1857 by Mayor Thornton, it served as a sewerage duct vent to allow the escape of noxious gases from the sewer (although it now ventilates the stormwater system, following works to separate Sydney's sewerage and stormwater systems).

It was built to ventilate Sydney's first planned sewerage system which was constructed by the Municipal Council. Sydney's first planned sewerage system consisted of five harbour outfalls.

The Hyde Park Obelisk was the first special sewer ventilation shaft built in Sydney and New South Wales, and is the only example of a sewer ventshaft constructed of sandstone within the Sydney Water system.

This technology was replaced with smaller, steel tube vents, which were used at intervals of approximately every 350 metres of sewer.

In 2014, the Hyde Park Obelisk was covered with a giant pink condom as a temporary installation to raise awareness about HIV, primarily in Sydney's LGBTIQ community. This led to some attempted outrage by Sydney newspaper columnist Miranda Devine, when she tweeted criticism of Sydney Water for apparently distasteful treatment of what she had incorrectly assumed to be a war memorial.