The managing director of Actew Corporation, Mark Sullivan, said workers could be stood down as early as next week as the company tries to prevent the $383 million project from a budget blow-out.

Mr Sullivan said employment costs of $150,000 to $200,000 a day were unreasonable during prolonged delays.

''Contractors may have to reduce the labour force, but clearly we're not going to be running three shifts, 24 hours a day, when nothing is being constructed,'' Mr Sullivan said.

''It's going to be a fair amount of time, so we're going to have to try and cut our costs … but what we don't want to do is lose our skills base. The damage to the dam will be covered by insurance, but delays are not covered by insurance. So we're responsible for delays.''

Last night, the surge of water spilling over the dam was 1.9 metres deep and Mr Sullivan estimated 60 to 80 gigalitres of water have flowed over the dam in the past three days.

''If the dam was built, it would have filled in about three days, but even the new dam would have overtopped with this rain … and we would have 78 gigalitres of water,'' he said. ''We've had two 100-year floods in three days.''

Mr Sullivan said a full expanded Cotter Dam would secure Canberra's water supply for 1½ years.

Yesterday's spills over the dam wall set a record with 39,000 megalitres a day flowing into the catchment.

''Our hydrologist is saying if we got dry weather, we'd see the flow go below the dam in two weeks' time. If we get moderate weather it could take five weeks before it even gets to a reasonable level below the new dam.''

Mr Sullivan said he expected work to restart in seven to 12 weeks.

Last night, the board of Actew met to get a briefing on the flood. ''We were hoping we'd be fairly close to having completed a full review of the cost and time of the project, but that's gone. Now we won't have it until one week after we get on top of the dam.''

Mr Sullivan said the flooding would delay the dam's completion by three months. The velocity of the water damaged scaffolding, formwork and earthworks downstream of the dam.

''I was hoping to get the wall complete by the end of August, but now I'm looking at the end of October or November.''

The ''Great Cotter Spill'' of 2012 has attracted a steady stream of tourists and Mr Sullivan urged Canberrans to watch the ACT's version of Niagara Falls online.

''The point where most people are parking is very dangerous, so they need to be extremely careful,'' he said.

But the lure of watching more than 20 million litres of water rushing over the dam each minute and dropping 40 metres almost crashed the company's website.

''It was going to bring down the whole site … we've had to extend the refresh rate, it's now 90 seconds rather than 40 seconds,'' he said.

The spill is expected to continue for at least 10 days.

''Possibly up to four or five weeks,'' Mr Sullivan said.