Placing of concrete ceased in March when the work site was flooded after heavy rain. Many staff were laid off and others found alternative work.

Above-average rain and an unexpected geological fault had already seen the cost of the dam increase from $363 million to more than $390 million.

The March flood has further increased this, however, the earlier than expected recommencement of work means the cost increase will not be as significant as first thought.

Though the 40m-high wall, about half its final height, was not compromised, much of the infrastructure and machinery had to be checked and timber removed from behind the new wall.

Actew announced today that inspections showed the new dam held up very well.

A spokesman said damage to the structure, equipment on top of the wall and tower cranes had been less severe than expected.

"Recovery work has also progressed very quickly thanks to the forward planning and efficiency of our construction partners," the spokesman said.

Ensuring the site was safe for work remained the top priority and would be essential when work resumed.

It was expected the placement of concrete for the dam would be completed by the end of this year and final structural concrete and commissioning to be completed in the first quarter of next year.

ACT Treasurer Andrew Barr welcomed the announcement.

"Returning to work sooner will help to ameliorate the cost increase arising from the record rains and flooding last month,” he said.

"I thank Actew and their partners for their hard work in assessing the damage and preparing to resume construction."