With the reservoir already at capacity, more than 97mm of rain pushed it past breaking point, sending water hurtling over the flood buffers that sit atop the dam walls.

Downstream, the excavation and construction works for Actew's huge Cotter Dam enlargement project were inundated by water flowing down the valley.

Project leaders were forced to call a halt to construction on Saturday, as safety teams assessed the possibility of soil erosion and prevented sediment run-off into the Murrumbidgee River.

But after a thorough safety inspection, the all-clear was given and work was able to resume yesterday morning.

Project leaders have been wary of flooding at the construction site and have implemented a range of measures to deal with excess rainfall.

The manager of the Enlarged Cotter Dam project, Ray Hezkial, said the flooding was the biggest threat the project had faced since it began in 2009.

''Unprecedented'' volumes of water had flooded the diversion pipes that usually dealt with excess water flows at the site.

For more on this story, including details of soil erosion concerns, see the print edition of today's Canberra Times.