A crowd in its hundreds has turned out to a community open day to learn more about the New Zealand Transport Agency's Maungatapu Underpass project.

The $45 million underpass being constructed on State Highway 29A will connect Welcome Bay with Turret Road and includes new cycle links plus a cycle and pedestrian overbridge.

Today's open day, which is based at Maungatapu School and is running until 3pm, features NZTA staff members sharing information on the project, plus activities for children and hourly guided walks around the construction site.

Earlier the rain stopped for long enough to allow the crowds to get a closer glimpse of the work on the underpass which is roughly at the half-way stage.

In fact, the organisers of today's event hoped to take small groups of about 20 people around the construction site, but such was the interest, some groups were more than three times that number.

One of the groups sets off from Maungatapu School.

A stop to view the underpass from the walkway between Maungatapu and Hairini.

NZTA highway manager Niclas Johansson says the underpass will separate state highway and local traffic, and in turn, reduce congestion at peak times and make travel safer for pedestrians and cyclists.

'We are just over one year in to this important build so it's time to let the community get up close to what is going on and speak to members of the team to find out more about this project.

'The project is a complex one on a busy road and we are making steady progress with the first 16 metre road bridge open and the 36 metre long, 34 tonne pedestrian bridge in use.”

A total of 120,000m3 or more than 4000 truck and trailer loads of fill was left to settle on the Welcome Bay side of the site between July and September of this year. While the second bridge is expected to be completed mid next year.

Niclas says the guided walks, which depart from the school on the hour, are 2km long and there's a free shuttle back to the school.

The New Zealand Transport Agency's Maungatapu Underpass Community Open Day is on until 3pm today. For more information visit: www.nzta.govt.nz/hairinilink

A stop to view the underpass from Welcome Bay Road.

A temporary walkway off Hammond Street with murals from Maungatapu School.

Namita Davey with her children, from left: Ahlea, 4, and Zacary, 6, stop to pose at the open day billboard.