On a handful of sunny days every year, Sydneysiders can get a glimpse of what the city looked like before it became the site of office towers and grid-locked traffic.

A tour group navigates through the Tank Stream, a heritage-listed stormwater tunnel under Sydney's CBD. Credit:Louise Kennerley

They have to venture down to a heritage-listed tunnel under Pitt Street, which was once a rich aboveground stream that flowed through the city and provided the Gadigal people with fresh drinking water.

The Tank Stream, as it is known, was heavily polluted by the early settlers and became an open sewer that was tunnelled over and eventually turned into a stormwater drain. It is now one of the last remaining constructions from the 1790s.

The tunnel is open to the general public on about four days every year - a tour only accessible via a lottery.