The force of the water flowing into the Queanbeyan River washed away a security fence after capacity was reached at 124,500 million litres and automatically opened the spillway.

Looking at a picture from 2007, Googong foreshore and Kowen Forest operations manager Geoff Webb said the area was almost unrecognisable. (Since December 2010 combined dam levels in the ACT have not dropped below 96 per cent.)

A boat ramp on the edge of the dam was completely visible during the drought. Yesterday just over a metre of the boat ramp, which is 40 metres long and drops 30 metres, lay above water.

Mr Webb said park managers made the decision to close roads to the dam for safety reasons.

During the last flood in 2010 rangers were overwhelmed by public interest in the spillway surge.

''More than 19,000 additional vehicles came to see the last flood event in 2010 and we weren't able to cope,'' Mr Webb said.

But this flood is much smaller. Flows over the Googong Dam Spillway were about 10,000 megalitres a day yesterday but in December 2012 they reached 98,000 megalitres a day.

Shortly before that flood, a $56 million project to upgrade the Googong Dam spillway was completed. Without that work there would have been a safety alert down river. Mr Webb said: ''Normally at this time of year, we're fighting fires so this is a nice change.''