The agency plans to clear a 4m-wide, 100m-long strip of native bush from the steep slope of Town Belt, between SH1 and the Water of Leith below, to allow drainage pipes to be drilled into the side of the hill.
The clear zone, running parallel to SH1 just below the highway, was needed to allow heavy machinery into the area, which would then be used to install drains under the highway.
The Otago Daily Times last month reported a section of the highway leading north out of Dunedin, near the Bank St intersection, was sliding away at an accelerating rate and in need of urgent engineering work.
NZTA staff suspected a build-up of moisture from rainfall and natural springs was to blame for saturating the area and causing the slip.
However, NZTA Coastal Otago area manager Roger Bailey said yesterday the agency had agreed to protect three kowhai trees in the clearance zone and, if possible, several large broadleaf trees and 10 to 15 other natives, mainly fuschias and pittosporums.
The trees had been identified during an inspection with Dunedin City Council staff, he said.
"There's a number of them we have identified to be left alone.
"The bigger, older, more mature ones, if we can avoid them we will do so," he said.
The council last month granted an easement over part of the Town Belt, allowing the work to be carried out, despite concerns expressed by council community and recreation services manager Mick Reece the work could leave an ugly scar.
Mr Bailey said the work was expected to begin early next year, take one or two months to complete and be followed by revegetation of the area with up to 170 native plantings.
The work would be carried out by a contractor, yet to be appointed, and would involve lifting drilling equipment from the highway down to the clearance zone, located just below the highway and running parallel with it, he said.
Contractors would then drill about 40 drains, each between 50mm and 70mm in diameter, 25m into the side of the hill, under SH1, to drain moisture away, he said.
The drains would link with another above-ground 100mm-diameter pipe, which would carry water down the hill and empty into the Leith, he said.
Mr Bailey said he was "100% certain" the extra flow of water would not cause erosion within the Leith, as the volume expected from under the slip was likely to be small, at about 0.02 litres per second.
That was 10 times less than the flow from a normal household shower, Mr Bailey said.
"It's absolutely a trickle we are anticipating. There won't be big volumes."
The drains would remain in place permanently, with an expected lifespan of "at least 20 years and hopefully longer", he said.
The visual impact of the work would be minimal, as the site was not an easily visible part of the Town Belt from many parts of the city, he said.
However, new plantings would take time to grow in and "a number of years" to completely restore the site.
"We are always careful of the environment, whether it's the Town Belt or whether it's just a normal piece of ground.
"The concern here is to mitigate the risk of a more severe failure of that slip, which would have such a dramatic impact on travel into and out of Dunedin."
Failure to secure the slip could see more severe damage to the highway, possibly leading to a partial closure, traffic diversions and repairs taking months and costing millions, he said.
That was a "worst-case scenario", and the most recent monitoring of the site indicated the slip was again slowing down - possibly because of drier spring weather, he said.
A sudden failure was not expected, but monitoring would continue and contractors would be asked to "swing into action quickly" if a more imminent threat to the road emerged, he said.
