State Highway 1 through Mangamuka Gorge could reopen to light vehicles by mid-October — but much depends on the weather.
NZ Transport Agency Northland manager Jacqui Hori-Hoult said work was about to start on repairing the most significant slip, on the northern side of the summit.
Seven other slips in the gorge triggered by the July 17 weather event had been cleared.
"Our priority is to get a solution in place that will secure the road that is left and open it in some form past the last big slip. That may involve convoys at set times and it will only be for light vehicles."
"We're looking to award a contract for this first phase of work in the first week of September and for work to begin as soon as possible after that. The work will take at least six weeks, depending on the weather."
The plan was to construct a wall of 20m concrete piles to support the road where the rock underneath has fallen away. The wall would be built from north to south to ensure the safety of workers and equipment.
"We're dealing with a very challenging and complex slip in a difficult environment. The continued rain isn't helping, but that's out of our control."
On July 17 the ground under the road fell about 300m to the river below, leaving the road surface unsupported to the road centreline.
In the first few days after the storm, engineers drilled into the road surface to test the stability of remaining rock.
They found the ground was unstable both under and above the road, sparking fears that repair work could reactivate old slips, Hori-Hoult said.
Continued heavy rain in the past month had resulted in parts of the hillside continuing to move. An ancient slip had also been above the road, complicating efforts to establish whether a new road alignment could be cut into the hillside.
"For that reason, building the concrete pile wall is a first phase in restoring the road to two lanes. We're essentially designing and building as we go and breaking the build into phases. For example, we'll have to install retaining walls above the road but we're still working out how much will be required. The team is really conscious of the natural environment and we want to preserve it."
For now SH1 remained closed between Victoria Valley Rd and Makene Rd.
Hori-Hoult said the rock structure through the gorge was weak and prone to slips so recent work to improve the resilience of SH10 as an alternative route was paying off.
She thanked motorists and local residents for their patience and said the agency was working as quickly as possible in difficult conditions.