Construction of the six reservoirs came with population growth and urban expansion. The monuments, built between the 1860s and 1930s, include stone bridges, dams, valve houses, staff quarters and aqueducts.

The oldest ones are in Pok Fu Lam Reservoir, completed in 1863.

More than half the monuments - 21 of the 41 - are at Tai Tam reservoirs, the second-oldest and four times larger than Pok Fu Lam's.

The minister said a five kilometre heritage trail was now open, with signs explaining monuments' functions and historic value. Guided tours will be organised for schools.

When it was proposed in 1873, the Tai Tam group of reservoirs was regarded as 'the grandest water scheme of its day'. It involved the excavation of the Bowen Aqueduct, a long tunnel through the hills to connect the catchment area in the south and the urban centre in the north of Hong Kong Island. The aqueduct, with its 21 arches, has been hailed as a piece of Victorian civil engineering heritage. The ambitious scheme was adopted only a decade later after a series of reports noted an alarming lack of hygiene and acute water shortages in Chinese districts.

Tai Tam also contains the only surviving waterworks-related chimney stack in the city, which was connected to the water pumping station, which in the early days relied on the burning of coal to generate steam to drive the pump.

Also present at the ceremony yesterday was a resident who had lived at the reservoir before the second world war. Helen Rigby, who lived in the staff quarters as a child between 1929 and 1939, is writing an autobiography about her life in the colony and her ordeal overseas during the war.

The Briton, whose father was an engineer in charge of the reservoir, said: 'I call it [the reservoir area] my home, and it is wonderful that it has now become a monument.' Rigby has contributed historical research to the Water Supplies Department which helped in the declaration of monument status.