Transmission Gully finally opening to traffic also means a stop to heavy fines that have been mounting for the road's builder because of delays.
A ribbon cutting ceremony was held on Wednesday morning and the road is due to open to the public today.
The $1.25 billion project was due to open in April 2020 but was riddled with delays, including five missed attempts at opening the road.
A fortnight ago, Waka Kotahi told contractor Wellington Gateway Partnership the 27km motorway between Wellington and Kāpiti had to be open by the end of the month.
Waka Kotahi board chair Sir Brian Roche said it was always known the engineering of the project would be challenging.
"But it's fair to say it's become a challenge in many other respects as well."
Those challenges translated into lengthy delays, which ultimately resulted in hefty fines for the road's builder, CPB HEB Joint Venture.
Waka Kotahi transport services general manager Brett Gliddon said fines had been accruing for months now and only stopped when the road opened.
He did not have the total penalty figure on hand but said it "rolls into the million - it's several hundred thousand dollars a day for being late".
"They're real penalties associated with the cost of the road because you've got to remember it's much like a mortgage where we've had financiers pay for the construction of the road and now we're going to pay it back over 25 years, so they're real, genuine costs."
There were also still a number of tasks to be completed, which were deferred to get the road open.
But Gliddon was confident about the road.
"I can assure the public that the road you're going to drive on in the next 24 hours is a safe road and is ready to have traffic on it ... there should be no concerns whatsoever in driving the road."
Minister of Transport Michael Wood said he had a "high level of confidence" there would not be errors in the road going forward.
"Always with these roads there are ongoing assurance processes. On a big project like this there might be issues which come up but we have a robust system in place to make sure that if there are defects that they get repaired."
While much would be said about the project's delays "today is primarily a day of celebration and acknowledging a very significant achievement", he said.
"Transmission Gully, quite simply, is one of the most significant and complex new roading projects ever undertaken in New Zealand."
A lot of earth was moved for the 27-kilometre project - 11.4 million cubic metres to be specific, with 865,000 cubic metres coming from the 800 metre stretch of Wainui Saddle at the northern end of the motorway.
Wainui Saddle is also the road's highest point at 253 metres above sea level.
There are 25 bridges and large culverts, all which have been built to withstand a 1-in-2500 years earthquake.
Te Ara o Toa bridge, near Whitby, is the longest (230m) and highest (60m) structure along Transmission Gully.
Waka Kotahi admits cell phone reception will be patchy on Transmission Gully.
Sections without coverage will vary between mobile providers.
Waka Kotahi said a suggestion parts of the motorway might have to be dug up to improve mobile reception was not correct as multiple ducts had been installed within the road corridor.
The agency noted all 111 calls are rerouted to any available mobile network and that Transmission Gully had a radar system that detected incidents anywhere along the motorway which will be monitored 24/7 by an operations team.
Waka Kotahi says it has been in discussions with mobile network providers.
When asked if the road was good value for money, Minister of Finance Grant Robertson said Transmission Gully ended up costing more than it needed to because the original establishment of the Public Private Partnership that financed the project "was flawed".
"While we have been clear in our views about what went wrong, today is a day to celebrate the fact that we are here, we have done this."
Prime Minister Jacinda Ardern said the motorway presented "a pathway that will be used by generations to come".
She said it would give 25,000 vehicles a day an up-to 15 minute shorter journey, which translated to a daily productivity gain of 1640 hours.
Local iwi Ngāti Toa blessed the road before Wednesday morning's opening ceremony, as well as gifting it a name in te reo: Te Ara Nui o Te Rangihaeata.