After months of delays caused by rain in early 2012, the last of 365,000 cubic metres of roller-compacted concrete will be placed, all but taking the dam wall to its maximum 80-metre height.

While this doesn't signal completion of the dam, it does mark the end of the largest section of works in what has been a turbulent year.

In March workers were stood down at the site after flooding set the dam's construction back several weeks.

In April, Actew Water revealed the project's budget had climbed to $405 million as a result of the record-breaking rain.

Eight months on, the organisation says the remainder of the dam's structure is on track for completion in Autumn and the project is keeping within its revised budget.

One million tonnes of rock were used to create the roller-compacted concrete that makes up the bulk of the new dam.

The rocks were mined and crushed on site, which water project manager Ray Hezkial said dramatically reduced the associated construction traffic and haulage kilometres on the neighbouring roads.

"This is a significant milestone, not only for the project, but also for the ACT and Queanbeyan community,'' he said.

''The wall has all but reached its full height of 80 metres, with the detailed, curved concrete cap on top of the dam as the next phase. Some work remains on the concrete structures running down the sides of the dam and in the stilling basin at its base.''

Mr Hezkial said 2012 had been challenging, but the project was finishing strongly and its revised schedule remained on track.

''The rest of the dam structure and related works will be completed by autumn next year, with the remainder of the rehabilitation, demobilisation and commissioning work due for completion in the winter.''

Despite drier conditions through winter and spring, an Actew spokesman said dam levels continued to hover between 95 and 100 per cent, but lower than average rainfall was expected through summer.

''Even though our water situation is quite positive it is still important to use water wisely now, and in the future, to sustain our water supply,'' he said.

But the spokesman said the combination of enlarging the Cotter Dam, and the already completed Murrumbidgee to Googong Water Transfer project and the Tantangara Transfer, as well high water storages and permanent water conservation measures, put the ACT in a good position.

''ACTEW Water is confident there will be no need for temporary water restrictions in the next few years. The suite of water security projects, combined with long-term sensible use, should keep us free of high-level water restrictions for prolonged periods for the next 20-30 years.''