But the Transport Agency now wants to extend that by more than half a kilometre.
AA spokesman Dylan Thomsen said with more growth north of Wellington and more traffic coming into the city, the extension would minimise congestion at a known pinch point.
"People might raise a few eyebrows when they hear about some additional work being required.
"But it's actually not that unusual with projects like this that run over many years for things to get changed and altered, as they get more modelling and updates on population growth and travel movements and things like that."
But Nick Leggett, who represents the country's truckies, said he was surprised the agency was only turning its mind to this now.
It is hoped the merge lane will ease congestion heading into Wellington's north. Photo / Ros Setford.
"NZTA I think have been looking for solutions over a number of years, and they've been thwarted sadly by regional decision makers, because they wanted to put an additional lane on State Highway 1.
"But it seems that this is a last-minute band-aid, because they've known about the problem for so long."
The agency said the area was identified as a problem as part of the operational readiness work it had carried out over the last year.
Leggett said about five years ago there was talk of having a new lane between where the two roads merge and running it all the way to the Tawa off ramp, to cope with the extra traffic Transmission Gully was going to generate.
The former Porirua mayor and head of the Road Transport Forum was worried the late design change would fall short.
"Is this the best fix or is this the band-aid? I'd like to understand a little bit more about what their modelling projects and if people using Transmission Gully on day one are going to be faced with compromised safety due to the number of vehicles coming off the Gully and needing to merge with the existing state highway."
Thomsen said when the new road opened people needed to be realistic.
The Transport Agency will call for tenders to build an additional 650m merge lane extension at Linden heading south, towards Wellington. Photo / Mark Mitchell.
"The traffic coming down Transmission Gully and meeting what is currently State Highway 1, at those peak times coming into the city we do have traffic and congestion in Wellington and that is still going to be there, regardless of Transmission Gully."
Even though the agency has not called for tenders yet, it wants the additional merge lane finished by the end of August.
Leggett warned there would be problems if it was not done when the road opened.
"If the road opens without this, it's going to make it far more difficult, and probably more dangerous, to exit off Transmission Gully and on to State Highway 1 at Linden," he said.
"There is doubt, I think, from a lot of people that Transmission Gully will indeed be finished by its new due date [September 2021]. There's a sense that it's going to continue to run over time."
The Transport Agency said over the coming months it would be working with councils, partners and communities to keep them fully informed of the plans and changes to the existing road network.
But Porirua Mayor Anita Baker said she knew nothing of the latest design update and would have appreciated it if the agency had come and spoken to her.
