Watch: Remarkable spectacle of Australia's 1930 built dam

Deep in the Adelaide Hills hidden among dense forest and bushland, there's a remarkable sight. For only the second time, South Australia's largest reservoir Mount Bold has been deliberately emptied.

Adelaide: It's a spectacle rarely seen in the life of this nearly 90-year-old dam.

Billions of litres of water released downstream from the Mount Bold Reservoir so engineers can assess the structure for a major upgrade.

"It is opportune to take this point in time to be able to do that and get in and do maintenance work that otherwise would have had to have been done by divers," says General Manager of the Dam Mark Gobbie.

Built-in the 1930s to store precious water for a dry but booming state the reservoir remains in good condition.

"Great care was taken and when we do our upgrade work, we also take great care, because dams are a little bit different from other infrastructure, they need to last a long time," says the Project Manager Eric Von Wielligh.

While crews are now assessing valves and pipework the plan is to start reinforcing the dam in 2022 so it can better withstand a large earthquake or a massive flood.

The water level is so low that workers need to be craned into the basin.

They've also been fishing.

Some native species have been found and crews have removed at least 27 tonnes of carp.

"So we're catching them to get rid of them, so we can do the work and not cause water quality issues," says Von Wielligh.

With a height of 50 metres and a capacity of 46-billion litres, the water from the reservoir reaches the taps of 450,000 homes and businesses in metropolitan Adelaide.

With much of the outflow stored further downstream and a little help from the city's desalination plant, South Australia Water says it's unlikely those taps will run dry.

"We're always hoping that our reservoirs fill naturally as quickly as possible. Certainly, the long term BOM (Bureau of Meteorology) forecast is probably more optimistic for the next 12 months than it has been for the last 12 months, but you never can quite tell what's going to happen," says Gobbie.

The reservoir should re-open to visitors in the next few weeks before water levels start returning to normal.

AP