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.
"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.