Transmission Gully is being officially opened today, though the public will not get to drive on it until tomorrow.
It is the sixth attempt to open the 27km motorway between Wellington and Kāpiti, and a ribbon cutting ceremony is being held this morning.
A fortnight ago, Waka Kotahi told contractor Wellington Gateway Partnership the motorway must be open by the end of the month.
The $1.25 billion project has been riddled with delays and was originally due to open in April 2020.
The end-of-month opening was described as "unbelievable" by Porirua mayor Anita Baker when it was announced, with her Wellington City and Kāpiti counterparts pleased about the social, economic and growth benefits Transmission Gully could provide.
There is not a specified time for the road to open to the public on Thursday which RNZ understands is because the Transport Agency does not want people queuing up and possibly causing congestion.
A separate ceremony to bless the link roads connecting Porirua suburbs Whitby and Waitangirua to the motorway took place yesterday.
Moves to speed up opening Transmission Gully were revealed in early March, when Waka Kotahi allowed road builder CPB HEB to defer the completion of 19 safety and quality assurance tests, as well as dropping some of the requirements of 30 other tests.
Waka Kotahi Transport Services general manager Brett Gliddon later said they were not "prepared to compromise on the road's safety" and, based on expert advice, were confident the road was safe.
Alongside the delays, the motorway has faced other controversies and issues for a long time.
Reports from last year showed attempts to fix parts of the road actually made it worse and that the project was flawed from the start.
The April 2020 date was pushed back a month because of the Kaikōura earthquake but by September 2019, Waka Kotahi was anticipating further delays as there were hold-ups to "critical construction.
There were also things like runoff from Transmission Gully being the likely cause of sediment rates more than doubling in Te Awarua-o-Porirua Harbour in the five years to mid-2019.
At the time, Waka Kotahi said it had an extensive runoff mitigation system but was also carrying out mass tree planting - measures, which combined, should offset the sedmient created during the building of the road within seven years.
The agency now says water quality and the surrounding environment will be in a better state than before the motorway was built within "less than 10 years" of Transmission Gully opening.
Environmental works are to continue into next year.
In 2017, Wellington Gateway Partnership sought new resource consents to move an extra 3 million cubic metres of earthworks after the original figure was underestimated.
At the time this brought it to a total of 9m cubic metres but Waka Kotahi now states the total earthworks for the project was "over 11 million", making it one of the largest volumes of earth to be moved on a New Zealand roading project.
In the mid-to-late 2000s and early 2010s, public agencies got the wheels of the project moving, while politicians debated whether the road should be built, its cost, and how it should be built. Resident groups opposed to the project were also established.
The final decision was a Public Private Partnership (PPP), with Wellington Gateway Partnership awarded the contract in 2014, a few months before the first sod was turned by then-prime minister John Key in September 2014.
In 2013, Waka Kotahi told a parliamentary committee that a PPP meant Transmission Gully could be built more quickly.
Documented public interest of a Transmission Gully route dates back as far as 1919, but it was not until 1987 that a study determined an inland route was preferred over upgrading the coastal route, which is now State Highway 59.
RNZ
