The expanded Cotter Dam is expected to overflow at about 8pm tonight releasing millions of litres of water to the Murrumbidgee River.
Water is rising against the partially completed dam wall, which sits 11m above the old dam, at a rate of more than half a metre an hour and millions of dollars' worth of machinery sitting on top of the wall will be submerged and is in danger of being swept away.
The new wall is holding about five gigalitres of water and the bypass pipe continues to channel water downstream. Its capacity is limited and the water level behind the new wall continues to rise. There are no plans at present to retain water behind the new wall after the rain ceases.
Actew project manager of the enlarged Cotter Dam project Ray Hezkial said the company had evacuated non-essential staff.
"The existing Cotter Dam is completely submerged and water is banked up to about 6 metres below the crest of the new dam wall," Mr Hezkial.
"Inflows rates are about 10 000 mega litres a day so if this continued for the next eight days it would fill the new dam up- if the wall was in place. At the current rate of rise we expect the water to be topping the new dam wall sometime this evening." Mr Hezkial said there was no danger of the dam collapsing.
"We may sustain some minor damage to the formwork, but the dam wall is quite resilient," he said.
Usually the area between the old dam wall and the new dam wall is free of water. In the past 24 hours this vast space has filled with water.
"Prior to the commencement of the rain event we had a total buffer of over 7 Gigalitres ,or 7,000 Megalitres, of air space. This has all but eroded in the last 24 hours and the volume of water that has filled the buffer equates to 17,500 swimming pools."
But after water levels recede and before construction can recommence the clean-up is expected to take more than two weeks.
Over the other side of Canberra, the Googong reservoir is full and up to 10 megalitres a day is flowing into the storage on the Queanbeyan River.
The spilling water is adding to the deluge along the river. But the flood is relatively minor compared to that of December 2010.
Actew managing director Mark Sullivan said about 10 times the amount of water had rushed over the dam in 2010. Shortly before that, major restoration of the spillway was completed.
Mr Sullivan said that work had proved essential. "We would have been on a safety alert had the spillways not been rebuilt," he said. "The threat of large pieces coming adrift was real."
Despite water spilling from all ACT dams, he said there was no threat of damage to them.
The only risk to the new Cotter Dam wall was minor flood damage if the water went over the top.
Despite the dramatic scenes of the submerged old Cotter Dam, people are urged not to visit the area.
The discovery trail has been closed and parking on the Brindabella Road would cause a hazard. Actew's dam cam can be viewed on its website, www.actew.com.au.
The new wall has recently reached about 40m of its eventual 80m height. Formwork on both edges of the new wall has been removed in preparation for the expected over-topping. Heavy machinery remains in place on the wall. Placing of concrete cannot begin for two or three days after rain.