Work will start on Monday on a temporary road to reconnect State Highway 4 - the Parapara - between Raetihi and Whanganui, with the aim of opening one lane as soon as possible.
The road has been closed since early October after it was significantly damaged by a major slip.
The site continued to move in the weeks following the slip, but geotechnical engineers last week said it had stabilised and was now safe enough for ground crews to begin work.
"Our crews are now preparing to enter the slip site, where we will begin drainage and revegetation before starting work to build a temporary alignment to reinstate the road," NZ Transport Agency (NZTA) regional transport systems manager Mark Owen said today.
"We know from feedback we've heard that it's important we restore access to the route, even if it is temporary and restricted. While we cannot say exactly what this temporary road will look like until we have fully assessed the site, we are aiming to open at least one lane to traffic as soon as it is safe to do so," Owen said.
"Early indications suggest that a full temporary road could be completed early next year; however, this is subject to further assessment as we work our way across the slip face.
"It is likely that once the temporary road is open, there will be limitations on its use. The road will be under constant observation and if deemed unsafe at any time, will be closed immediately."
There are several other problem sites along State Highway 4 between Matahiwi and Whanganui, where in some places the road is down to one lane.
NZTA has a summer work programme for these sites. Some are subject to final consenting and land owner agreements, so the programme is likely to take two years to complete.
"At the same time, we will be continuing to investigate long-term solutions to reinstate the road," Owen said.
"This will take more time as we work through options to strengthen the highway's resilience so that it can better withstand the challenges of the unique topography and severe weather events.
"Locals, road users and businesses have been very patient and understanding and we are committed to restoring access to this critical route for them as soon as possible."
NZTA's recommended detour route is via State Highways 1, 49 and 3, which will add at least an hour to the journey between Raetihi and Whanganui.
The agency says alternative routes, including Whanganui River Road and Fields Track, are not suitable for heavy vehicles or large volumes of traffic.