The new Transmission Gully motorway will open up the Wellington region and bring in more business, a local mayor says.
Prime Minister Jacinda Ardern, National Party deputy leader Nicola Willis, and MPs and mayors from across the region were at the ceremony.
Ardern said with a shorter journey each day for about 25,000 vehicles, daily productivity would increase by 1640 hours, "meaning less time spent in cars and more time at work and with family".
Transport Minister Michael Wood says it was one of the most challenging road projects in New Zealand.
Porirua mayor Anita Baker was one the attendees and said Transmission Gully was "huge" for the region.
The city had three entry-points onto the new road, which meant people could travel to and from Porirua very quickly, and it would bring in more business, she said.
Baker said the old State Highway 1 was constantly congested. "A one way road with a median barrier, accidents all the time," she said.
Now it would become a local road and traffic would move easily on the new motorway.
The mayor said she did want to waste time or money on an inquiry into the delays.
The $1.25 billion project has been riddled with delays and was originally due to open in April 2020.
A fortnight ago, Waka Kotahi told contractor Wellington Gateway Partnership the 27km motorway between Wellington and Kāpiti must be open by the end of the month.
Kelly Fox, who runs the Traffic Updates - Horowhenua Kāpiti Wellington Facebook page, has been providing updates on the road for the past three years.
She was taken on a tour of the road earlier this week and told Morning Report the "wide open" highway felt safe to drive on.
"It's one thing to hear about it all on paper ... but to actually drive on it and experience the drive times - we were in Paekākāriki one minute and then not nine, 10 minutes later we were half way down to Porirua and that wasn't even going 100km/h.
"So the drive time, the fact that there's another road to and from the city in case of incidents - I think it's going to be absolutely wonderful, and that's an understatement, for our region."
Morena from Transmission Gully. Crazy to be here for the opening of the motorway having grown up in Paekakariki, where it was a hot topic since childhood. @NZMorningReport @radionz pic.twitter.com/b9b5NZbSSF— Jake McKee (@journjake) March 29, 2022
Road transport advocacy group Transporting NZ chief executive Nick Leggett said it would change traffic flows, and make the Wellington region more resilient in extreme weather events.
"Even this year there have been days where peak traffic north to the Kapiti Coast has been stopped because of accident.
"Closer to winter things like even a branch over a railway line can clog the whole transport network across Wellington.
"So this road is going to give us resilience ... it's going to make traffic more reliable and therefore make it easier for people to get around."
Roads were not the total solution and there should be continued investment in public transport, he said.