Wet weather is likely to hamper efforts to work out a solution to the State Highway 4 Parapara slip.
NZTA says wet weather forecast this week is expected to make the large hillside slip worse.
The road linking Raetihi and Whanganui has been closed for two weeks and the slip site itself is also closed with no public access permitted until it stabilises.
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NZ Transport Agency journey optimisation manager Lance Kennedy said the health and safety of road users and workers is their top priority.
"Any rain in the next few days could exacerbate the slip movement and further hamper our efforts to enter the site safely," Kennedy said.
"In the meantime, we are monitoring and assessing the site using drones."
The initial findings of a geotechnical investigation into the landslide near the Matahiwi Track revealed that work to reinstate the road will be significant and complex.
Geotechnical engineers told NZTA officials that more work would be required before they could provide a possible timeframe for repair or alternate course of action.
Kennedy said NZTA knows the road closure is having a significant impact on local residents and businesses.
"We are working hard to find a safe solution for reinstating the road as soon as possible," he said.
"Given the size and complexity of the slip, this will take time and a concerted effort, but it is a high priority for the Transport Agency and we will keep the community updated on progress."
The road will likely remain closed for a significant time and motorists are advised to plan ahead.
The detour route recommended by NZTA is via State Highway 1 and State Highway 49, which will add at least one hour to the journey.