The Tank Stream, which runs under Sydney from Hyde Park into Circular Quay, is the reason Australia's first city was located on the picturesque harbour.

In 1788 Captain Philip sent out an expedition after no fresh water could be found around Botany Bay, and it was decided the spring-fed stream could provide fresh water for the new colony.

The drought in 1790 led Governor Philip to order three storage tanks to be constructed in the sandstone beside the stream, giving the water course its name.

Bridge Street was named after a timber bridge that crossed the stream connecting the governor and officials on the eastern side of the stream with the soldiers and convicts who were originally housed on the western bank of the stream.

Yvonne Kaiser-Glass points out the spring water still trickling into the stream. ( 702 ABC Sydney: John Donegan )

The colony eventually outgrew the stream's capabilities and new water sources were found, as the growing population caused the water source to become polluted.

The heavily polluted Tank Stream was covered in 1879 at a cost of 1,517 pounds, and was transformed into the first sewerage run in Sydney.

Fast forward 136 years and the Tank Stream is now a popular tourist destination with Sydney Water allowing four days of tours each year.

More than 5,000 people subscribe to a random ballot for the 196 sought-after places on the tours, with some keen adventurers entering the ballot nine times before winning a spot on a tour.

Colonial era workmen marked the stones they laid with a signature. ( 702 ABC Sydney: John Donegan )

The sounds of a bustling metropolis can still be heard as you descend under the city via a steep ladder.

Sydney Water archaeologist Yvonne Kaiser-Glass runs these biannual tours through a 60-metre section of the Tank Stream, which exposes artefacts of Australia's colonial history and development.

The thud of traffic driving over the manholes covering access points to the Tank Stream punctuates Ms Kaiser-Glass's commentary.

"Stepping over the line from the reinforced concrete onto the sand stone floor we step back over 220 years," Ms Kaiser-Glass says.

In the drought of 1790 Governor Phillip commissioned a team of convicts to shape and smooth the stream floor to stop the water pooling.

The last fish in the Tank Stream is a discarded soy sauce container washed from the street by storm water. ( 702 ABC Sydney: John Donegan )

"There are peckings visible now which brought the high spots down and sandstone inserts to raise the low areas to stop the stagnation of the water," Ms Kaiser-Glass points out.

The importance of the stream as a life source predates colonisation, with evidence that the Gadigal people used the stream as a source of freshwater and food.

Aboriginal artefacts including tools fashioned from river bed pebbles were found in the bed of the stream during twentieth century excavations.

The Tank Stream tour ends at the narrow oviform section dating to about 1870. ( 702 ABC Sydney: John Donegan )

Natural spring water still flows into the Tank Stream at one point under the appropriately named Spring Street.

The stream now carries mostly storm water, and still flows into Circular Quay near Pier Six.