The design of Scrivener Dam has to be changed because dangerous corrosion on anchor bolts was virtually impossible to discover.
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The National Capital Authority revealed on Wednesday the work was on time and would come in millions of dollars under budget.
The authority will take MPs from the federal public works committee on a tour of the work on Friday.
A new safety contractor uncovered the corrosion in the bolts in 2011, forcing the authority to lower the lake level by 500 mm to undertake the repairs.
NCA chief executive Gary Rake says work on the first of the dam's five gates will be completed by the end of this month. "That gate will be fully back in service with a design life of at least 100 years by July 8 and we will move straight into working on the second gate,'' he told a parliamentary committee. "We initially estimated the budget would be $14 million for this project, we've revised that down to $8 million.
"That saving has really come about by taking extra time to analyse the problem and design it well.''
Mr Rake and NCA chairwoman Shelley Penn appeared before the National Capital Committee on Wednesday for the authority's biannual public hearing.
Mr Rake confirmed a change in the safety contractor for the dam led to the discovery of the problem.
"In 2011 they undertook a more extensive review and found early signs of corrosion," he said.
"What we discovered was that this corrosion probably started the first time the water flowed over the dam which would have been 1964 and had been undiscovered since. We had to put in place some pretty urgent safety measures to make sure we protected community safety.
"That gave us time to remediate the problem and the two things we are doing is fixing the heart of the problem so that the dam is safe for another 100years but we’re also making sure there’s no prospect of an undiscoverable problem emerging like this.
"The way the dam was built in the early ’60s made it virtually impossible to have discovered this problem earlier."
Mr Rake said work at Bowen Place was proceeding in three stages, with the first being the recent relocation of underground gas and electricity mains.
