Close to 100 people who were working on Transmission Gully are overseas unable to return to the project site under Covid-19 border restrictions.

There is growing concern the now billion-dollar project could be significantly delayed, after having already been behind schedule resulting in a budget blowout.

The 27km four-lane motorway is being built through a public-private partnership, the Wellington Gateway Partnership (WGP), with CPB Contractors and HEB Construction sub-contracted to carry out the design and construction.

The massive lower North Island motorway was meant to open on November 1 this year, but following five weeks of lockdown, will undoubtedly be delayed.

Work resumed on several sites in the project on Wednesday with others set to follow as crews are re-inducted to ensure compliance with new Covid-19 protocols.

NZ Transport Agency project delivery senior manager Andrew Thackwray said 182 people were working across the four reopened sites with another 59 people working from home.

However, a significant portion of the site engineering and supervision team, about 80 roles, and some of the operations personnel, are at home overseas after leaving the country due to the uncertainty of Covid-19.

They include stabilisation crew members and paver operators, Thackwray said.

"Work is currently under way to develop a new delivery plan as quickly as possible, utilising local resources. Some people and equipment are unable to be utilised until this revised plan is finalised."

NZTA is negotiating with the joint venture to confirm a new completion date for the project, which will now likely extend into 2021, Thackwray said.

The Transport Agency understood the builder was making adjustments to the work programme after the lockdown period in line with the change of season.

"Many of the activities due to be completed were not scheduled to be worked on at the onset of winter so this requires a major change in how work on Transmission Gully will need to be delivered," Thackwray said.

These further details from NZTA on the state of the project follows a press release issued this morning by Road Transport Forum chief executive Nick Leggett calling on the Government for urgent clarity on Transmission Gully's completion date.

Leggett said he has been contacted by several people associated with the build who had suggested the timeline for completion could be extended by two years.

"In the final straight of construction of such a large project, which was due to be completed by the end of this year, it is a worry to hear that contracts for supply and equipment have been cut, and workforce numbers reduced.

"If Transmission Gully can't be completed due to labour and budgetary issues, it undermines confidence in the Government's ability to deliver on any other infrastructure projects."

National Party transport spokesman Chris Bishop also called for urgent clarification on the project.

He said some delays due to Covid-19 and the five-week lockdown were understandable, but a two-year extension to the timeline would be excessive.

"It's extremely concerning at a time when we need to invest and build in vital pieces of infrastructure to get the economy going again."

Transport Minister Phil Twyford said the Government was committed to completing Transmission Gully as soon as possible and he expected NZTA to update the public with a new completion date in a similar fashion.

He said there were hundreds of people working hard on Government projects across the country.

"The implication that they aren't getting on with it and can't deliver because of unsubstantiated rumours about one project is ridiculous," Twyford said.

"We're working with the industry through the Construction Sector Accord to make sure we have the workforce needed to build economy-boosting and job-creating projects around the country."

Earlier this year it was revealed the cost of Transmission Gully blew out to more than $1 billion after NZTA agreed to pay the contractor another $190.6 million in a settlement.

That was only half the amount the contractor wanted.

The joint venture sought relief after work was delayed by the 2016 Kaikoura earthquake, and flooding around the same time. It was granted a 20-working day extension last year.