Almost a month after State Highway 4 slipped away, NZ Transport Agency assessors are able to begin on the ground investigations into what caused the road to crumble away.

The unstable ground has meant geotechnical assessments have been limited to drones since the 400m-wide slip in early October.

NZTA regional transport systems manager Mark Owen said work will begin on short-term options for reinstating the road.

"We know how important this road is to locals, road users and businesses," Owen said.

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"It's clear from feedback we've received that even temporary, restricted access along this route is vital.

"Finding a safe solution for reinstating the road as soon as possible is a top priority.

"Until our ground crews have fully assessed the site we won't know exactly what this will look like or how long it will take, but we expect to be able to announce a temporary option soon."

Detours are in place while NZTA assesses the massive slip on SH4. Photo / Bevan Conley
Detours are in place while NZTA assesses the massive slip on SH4. Photo / Bevan Conley

Owen said public safety is paramount and any temporary option would be closed immediately or at short notice if it is deemed unsafe.

"Due to the scale and complexity of the slip, a long-term solution will take more time.

"We will be approaching this project as an opportunity to strengthen the highway's resilience so that this critical route can better withstand the challenges of the unique topography and severe weather events."

NZTA is working with Ruapehu District Council to install signs to remind travellers that Raetihi is still accessible and open for business.

The recommended detour route is via SH1, 49 and 3, which adds about an hour to the journey between Raetihi and Whanganui.

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The alternative routes via Whanganui River Rd and Fields Track are not suitable for heavy vehicles or large volumes of traffic, and NZTA is advising the state highway network is the safest and fastest route available.