The New Zealand Transport Agency has confirmed Transmission Gully will "undoubtedly" be delayed, with one of the key Covid-19 impacts on the project being workers returning home overseas.
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 road was meant to open on November 1 this year, but the fallout from Covid-19 meant it would "undoubtedly be delayed", project delivery senior manager Andrew Thackwray said.
"To what extent, we're not entirely sure, we're just working through the new programme with the builder to determine just how much an impact Covid-19 has had on the programme."
The three biggest impacts coronavirus has had on the programme are the closure of the site, workers leaving to return home overseas, and inter-regional workers' inability to travel for a period of time, he said.
Thackwray could not provide figures regarding how many people had left to head overseas.
Other roading projects around the country were not affected in the same way, as they were largely locally delivered, Thackwray said.
The massive lower North Island project is all too familiar with being delayed.
Earlier this year it was revealed the cost of the project 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.
NZTA considered the "financial relief" was provided in acknowledgement that the issues were out of the control of CPB HEB JV and not something it could have reasonably anticipated.
Transmission Gully is one of 44 state highway projects around the country which are able to restart in alert level 3.
Meanwhile, KiwiRail will resume work on almost all of its projects including the Kaikōura rebuild and Wellington metro upgrade.
It means more than one thousand road and rail workers are back at work to help kick-start the post-lockdown economy.
Transport Minister Phil Twyford stressed moving to alert level 3 did not mean a return to pre-pandemic ways of working.
"Strict health and safety protocols are being followed at every site to keep workers and the public safe. These include restricting access to sites, requirements for workers to maintain physical distancing and the use of additional protective clothing.
"It's too soon to say whether Covid-19 has had any effect on construction timeframes on individual projects. My expectation is that project teams will look at accelerating works where possible," Twyford said.