It quietly winds its way through Sydney's heart - underneath offices and crowds, landmarks and traffic - to emerge just handful of times a year as one of the most sought-after tickets in town.

The Tank Stream was opened up to a lucky few on Sunday - more than 226 years after Captain Arthur Phillip identified it as the lifeblood of the future colony, which, in turn, soon contaminated the fresh water source and eventually drove it underground.

The Sydney Tank Stream is still an active stormwater drain. Credit:Steven Siewert

About 160 people from among the thousands who typically try for a spot on a Tank Stream tour were granted access to a small section of the tunnel near Hunter Street, where the pick marks left by convicts during a 1791 drought can still be seen hewn into Sydney's bedrock.

"It's one of the least accessible tourist spots in Sydney," said Sydney Water's archaeologist Yvonne Kaiser-Glass, who led the tours organised for Sydney Open through what now largely functions as a stormwater channel.