Earthquake strengthening work on the Karori Tunnel will not be completed until early March, Wellington City Council says.
Work on the tunnel, built in 1900, began last February.
It included rebuilding the tunnel's entrances, recreating the historic facade and building a sprayed-concrete retaining wall on the Kelburn side of the tunnel.
About 200 holes have been drilled into the hillside and long steel rods inserted to securely anchor the new tunnel entrances in place.
The council's city networks manager Stavros Michael said most of the work on Karori Tunnel was completed and it was operating normally most of the time.
"It's difficult to do major structural work like this on a busy route," he said.
"The tunnel is used by about 114,000 vehicles a week and our priority has been to complete the work safely and minimise disruption.
"We still have some finishing work to do, including plastering on work on the new tunnel entrances, [but] the big new retaining wall on the city side of the tunnel is complete," he said.
"All going well, we expect the last of the work to be completed by early March. The project has made the route to and from Karori considerably safer."
The project was supposed to be completed by the end of last year, but was delayed because of a slip in October.
Engineers had to investigate, design and construct a new retaining wall before work could continue, Mr Michael said.
"The work has taken a bit longer than we originally expected, but we are pleased with how things have gone and would like to thank Karori people for their patience."
The next tunnel to be strengthened is the Hataitai bus tunnel, which was built in 1907.
Mr Michael said the tunnel was being assessed so the council could work out what needed to be done and how to do it.
He said it was too soon to say when work would start or how long it would take.
- The Wellingtonian