Hundreds of people came out to the Maungantapu Underpass open day to learn about, and take a guided walk of, the $45 million project.

The underpass was expected to be completed by April 2018 and would connect Welcome Bay with Turret Rd, and separate State Highway 29A traffic from local traffic.

It also included new cycle links and a cycle and pedestrian bridge that had already been completed.

NZTA project manager John McCarthy said the open day at Maungatapu School was for locals to see the progress and understand all the activity going on right under their noses.

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"Everything is going to plan and we're happy with the progress. The public have been very patient and understanding with the changes to traffic," he said.

Mr McCarthy said an interesting method of construction was being employed - building from the top down.

First trench walls were built, then bridges connected to the walls. The earth below the bridge was then dug out. The floor was constructed and tied to the trench walls.

"The first bridge is already done and the second should be done by May next year," Mr McCarthy said.

The project had not been without challenges. The huge amount of dirt that needed to be brought in, meant a lot of trucks were travelling through the area causing traffic congestion, however, that phase was completed.

The workers also found asbestos near the Old Scout Hall, which was a surprise.

The open day informed people about the environmental effects of the huge roading project and showed videos and models of what the finished underpass would look like.