The ACT's half-completed Cotter Dam overflowed last night, spilling millions of litres of water down to the Murrumbidgee River as record-breaking rain continued to pound the region.
The spill began at about 7.15pm, threatening building equipment and causing further delays to the $363 million project which has been plagued by bad weather since construction began in early 2010.
Behind the dam wall, which is standing at 40 metres - half of its finished height and 11 metres above the old dam wall built in 1916 - the water level was rising at a rate of more than half a metre an hour throughout the day, completely submerging the old dam.
Despite an estimated flow of 27,000 megalitres of water through the Cotter valley, Actew project manager of the enlarged Cotter Dam project, Ray Hezkial, said there was no danger of the dam collapsing.
Planning for the spill had been undertaken and workers had yesterday removed some formwork in a bid to channel the overflow away from the millions of dollars worth of machinery sitting on top of the dam wall.
''We've removed a section of formwork so that it goes through that gap and shoots through down into the river,'' Mr Hezkial said
''There is no way to move those vehicles so our flood management plan has always been to tuck all the equipment we can up tight against the form. The forms give us some form of protection.''
Mr Hezkial said it was unlikely the machines would be washed away.
''We don't believe so, given their mass, but you never know. What's more likely is that the formwork will get stripped if the velocity is high enough and the machinery will be waterlogged, but the dam wall is quite resilient.''
He described the downpour as a ''severe flooding event'' and said it was the quickest rising water levels he had ever seen.
''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.''
Usually the area between the old dam wall and the new dam wall is free of water.
But in the past 24 hours this space has filled. Mr Hezkial said that before the start of the heavy rain, ''we had a total buffer of over seven gigalitres, or 7000 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.''
Mr Hezkial said he did not know the exact cost of stopping work for a day. But he said it would be in the order of about $200,000.
But after water levels recede and before construction can resume the clean-up will take place.
''The mopping up; that takes a significant amount of time. No work can be done when it's raining,'' he said.
When construction of the new dam began, it was expected to be completed by about this month. But the new wall is now expected to be completed by about September and the entire project by the end of the year.
The project began with an overall budget of $363 million. A geological fault at the base of the wall delayed construction by several months and cost about $10 million.
In July last year, Actew managing director Mark Sullivan said contingencies meant the project remained on budget but remediating the fault, on top of the previous year's flooding, had knocked the budget into critical.
Yesterday he said: ''Clearly, the budget is under pressure. We can't beat rain. Rain has been an absolute killer for us. We are still confident we can complete the dam this year.''
A renewed schedule and budget was almost complete. ''The budget will not be blown totally,'' Mr Sullivan said.
Before construction began, Mr Sullivan said building the dam was sure to break the drought.
Meanwhile, 400mm and rising of water was surging over the Googong Dam on the Queanbeyan River. Mr Sullivan said this would cause moderate flooding. However, it would be much less significant than the flood of December 2010, when about 10 times the amount of water had gone over the dam.
Shortly before that flood, a $56 million project to upgrade the Googong Dam spillway was completed. Mr Sullivan said yesterday that without that work there would have been a safety alert.
''The threat of large pieces coming adrift was real. We have a very secure, safe dam.''
Unlike December 2010, this flood had been relatively moderate. The dam mitigated against further damage by slowing the water.
Mr Sullivan said he was more frustrated than concerned over delays to the new Cotter Dam. ''There is a lot of pride in the structure and pride in the way it has been managed. But it is very difficult to beat the elements.''
Two clear days after rain were needed before concrete placement could resume. About 60 per cent of time had been lost in December and February.