The Lerderderg River diversion tunnel, known as The Tunnel, is located on the Lerderderg River

Lerderderg River

The Lerderderg River is a river in Victoria, Australia. The river begins in the Lerderderg State Forest and ends when it merges into the Werribee River just out of the town of Bacchus Marsh....

  approximately 25 km north-west of Bacchus Marsh

Bacchus Marsh, Victoria

Bacchus Marsh is an urban centre and suburban locality in Victoria, Australia located approximately west of Melbourne and west of Melton. The population of the urban area is estimated at over 17,000 people, while the central locality is home to 5,566 people...

, Victoria, Australia

Australia

Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

. The river flows around a spur in a horseshoe bend. The tunnel was driven through the spur diverting the river and allowing the exposed river bed to be sluiced for alluvial gold.
The tunnel was dug during the Victorian gold rush

Victorian gold rush

The Victorian gold rush was a period in the history of Victoria, Australia approximately between 1851 and the late 1860s. In 10 years the Australian population nearly tripled.- Overview :During this era Victoria dominated the world's gold output...

, most likely between 1855 and 1870,
and is one of around thirteen river diversions surviving from that era.
The site is within the Lerderderg State Park

Lerderderg State Park

Lerderderg State Park is a 14,250 hectare park located between Bacchus Marsh and Blackwood, an hour's drive from Melbourne, Australia...

. It is a 1.5 km walk along Byers Track upstream from the O'Briens Crossing Picnic Area.