The Great Cotter Spill of 2012 was steadily attracting plenty of onlookers after the record-breaking rain of the past week forced water to spectacularly thunder over the new dam's wall on Thursday night.

At its peak the partially completed dam had more than 20 million litres of water rushing over the crestline each minute.

The roar of the water making a 40- metre drop could be heard 5 kilometres away at Uriarra Village and hundreds of Canberrans drove out to vantage points along Brindabella Road to see the spectacular spill as record-breaking rain pounded the region. The highest inflows into the catchment were 29,000 megalitres a day and at that rate, if the new dam was fully completed, it would be filled in two and a half days.

Campbell couple Jennie and Peter Radtke said they came to see the power of nature.

"It's a lot of water, it's very powerful and they haven't even finished building it," Mrs Radtke said.

The velocity of the water pushed scaffolding and formwork downstream and Actew manager of the enlarged Cotter Dam project Ray Hezkial said it would be at least two months before work on the dam could recommence.

"We expect, based on the flows currently coming through, that it could take anywhere between two to four weeks before the level drops sufficiently to even permit access to the site for the commencement of a damage assessment," Mr Hezkial said.

"I would conservatively estimate, based on what is visible of the dam wall, that it would take at least four weeks to clean up and repair-replace damaged plant and equipment before concrete placement could recommence."

The spillover has damaged million of dollars of equipment and caused further delays to the $363 million project which is running over schedule and budget.

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More reports – Pages 4, 5