However, a substantial number of the original anchor bolts were left in place - encased in concrete - and new anchor bolts and associated connections were retrofitted to replace them.

The new bolts were drilled about five metres deep into the dam wall.

The new anchor bolts have been installed in a corrugated plastic sheath and cement grout to prevent corrosion, extending their lifespan for another 100 years.

By securing the floodgates to the dam wall, the anchor bolts allow the gates to pivot about their bottom edge to regulate the water level of Lake Burley Griffin, including being opened during floods.

The authority's chief executive Malcolm Snow says the project's engineers designed a very efficient solution to a difficult problem.

''I was just absolutely blown away by the technicalities of it … it's a very elegant solution,'' he said. The project cost $7.4 million, about half the original estimate.

An NCA contractor identified corrosion in the anchor bolts in 2011, forcing the authority to lower the level of Lake Burley Griffin by half a metre.

Former NCA head Gary Rake said after the discovery, no one could rule out the floodgates failing due to the bolt corrosion.