Bowen Drive will remain partially closed and subject to a 40km/h speed limit around the construction zone until the expected opening in early September.

Construction of the underpass near Lake Burley Griffin started nearly 15 months ago and was anticipated by the National Capital Authority to be finished in March this year.

Scaffolding has now been removed from the underpass, revealing the polished precast concrete walls – and literally some light at end of the tunnel for the long-running project.

National Capital Authority chief executive Malcolm Snow said it was a "significant milestone in that we've broken through as it were".

He said the project was now expected to finish in September.

"What we're trying to do is make up some lost time due to the significant periods of wet weather and we've also had what's called latent conditions, where underground services were not where they were surveyed as being," Mr Snow said.

The National Capital Authority was working with the contractor Woden Constructions to improve on that opening date.

Mr Snow said the delays had not blown out contingencies within the $10 million budget, bankrolled by the Commonwealth.

"We're confident we can still bring it in on the approved budget," he said.

"There have been some cost savings but also equally some things that have come to light, such as the services, where we've had to dip into the contingencies. The contingency was healthy but it was all within the $10 million budget."

Mr Snow said he was disappointed the project was delayed but believed it was worth the wait to fix what has been a dangerous crossing.

Cyclists, joggers and walkers on the popular bridge-to-bridge walk around Lake Burley Griffin have for years had to scramble to avoid being hit by traffic on Bowen Drive which sweeps under the Kings Avenue Bridge.

"I am disappointed by I can assure you we've taken all necessary steps to see how we can claw back some time due to things beyond our control," he said.

"I can understand people, particularly commuters in the morning being frustrated by how long the project is taking, I can assure you all the improvements from this major public space redevelopment will be obvious and people will forget their trip time was extended by two minutes. It's a small inconvenience for what will be a very large public gain."

Mr Snow said the authority would negotiate with the ACT government about the future speed limit on that section of Bowen Drive.

"We would certainly favour keeping it at a lower speed limit there," he said.

"Even though we will now be able to avoid that previous conflict that was there between pedestrians, cyclists and cars, we are still keen throughout the national triangle to keep vehicle speeds low."

The underpass has been a long time coming, first announced nearly four years ago and then thought to be open in time for Canberra's centenary in 2013.

The authority put the project on hold in late 2011, as it juggled its budget to pay for unexpected major engineering works to Scrivener Dam

Parliament then approved the latest design in November, 2012.