Both routes are the western-most roads of the options and essentially the shortest route.
An NZTA media spokesperson said it was a conscious decision to inform all property owners affected by the decision by letter or telephone prior to the official announcement, which was expected today.
She said NZTA was aware of how emotive the decision might be for those affected by the new route decision.
Letters started arriving on Monday night, hand-delivered, while some property owners got phone calls from the agency. Copies were posted on discussion group pages on social media that night.
The letter said: "The preferred corridor delivers strongly on safety and resilience, which are high priorities for many in the community...it will also provide better connections to urban areas and planned growth areas".
The much-anticipated official decision would come in a media release to all agencies sometime on Wednesday.
Horowhenua mayor Michael Feyen said he was very pleased NZTA had made a decision as he feared the project had been put on the backburner.
"We've always had positive dialogue with NZTA and have always been clear in what we want to achieve," he said.
"I have to say they are doing the best they can. They have taken so much care with consideration to environmental, economical and social impacts."
He was thankful to mayors from neighbouring districts who had kept the need for a new road high on their list of requirements whenever there was an opportunity when dealing with NZTA.
"There were six of them and I have to thank them for always keeping it at the top of the page," he said.
"There are other roading projects going on and at one stage it looked like it might disappear."
He said there still a lot of work for NZTA following the announcement and did not expect any real work to begin for "at least two or three years".
They were working within a wide section of road identified in the plans that provided a "corridor" for the final route.
The corridor was a much wider section than required for the new road to allow for adjustments as the exact route is finalised.
Mayor Feyen said as the bypass would improve road safety and save lives in the future, the immediate danger on the current motorway for motorists remained.
He said there were so many heavy traffic vehicles using the highway and an increase of farm machinery operators this time of year that her urged caution over the festive season.
"I just want to tell people to just be bloody careful out there," he said.
The letter to property owners said NZTA would continue to finalise the exact route and there may be slight changes, which would mean the way some properties were affected could change.
NZTA wanted that process completed by 2020.
"We will also be seeking to understand the effects on individual properties...such as possible noise and visual impacts and we will consider how these can be avoided, minimised or otherwise addressed," the letter said.
NZTA was to hold meetings in Levin and encouraged any affected land owners to get in touch with them.
Ōtaki MP Nathan Guy said he was pleased the affected residents now have certainty about the road.
"I sympathise with those impacted by this decision and acknowledged the emotional stress this will bring."
He said he would push hard for NZTA to ensure there was a pot of money available to buy out those who wanted out quickly.
"NZTA needs to show compassion to those directly impacted. There is still much uncertainty on funding and our local community is up in arms about the decision the change the project from four lanes to two lanes. This is really short-changing us in favour of Auckland and trams."
"I challenge NZTA do more for safety, like a flashing sign urging caution on those narrow railway bridges, immediately ahead of the busy holiday season. The last few weeks have been horrific on this stretch of state highway."
