The size of the Parapara Rd slip is similar to large slips in the Manawatū Gorge in 2011, NZTA says.

It revealed the initial findings from a geotechnical investigation into the State Highway 4 slip today, confirming work to reinstate it will be significant and complex.

Geotechnical engineers confirmed more work needs to be done to fully understand what repairs are required and the approximate timeframe for reinstating the road.

NZTA journey optimisation manager Lance Kennedy said at its widest point the SH4 Parapara Rd slip is about 400m wide, 200m high and up to 50m deep.

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"Estimates are that there is around 30ha of material that could slip, of which around 15 to 20ha has already moved," he said.

"To put this into context, the size of this slip is similar to the large slips in the Manawatū Gorge in 2011.

"The site is still highly unstable and remains fully closed off. It is continuing to move and any rain in the next few weeks is likely to exacerbate the slip movement."

With the road and the slip site closed off, geotechnical engineers have been assessing the damage remotely using drones since last Thursday.

The road has been closed since Wednesday, October 2 after an underslip that created an unstable surface and then quickly deteriorated further.

The next day, a huge crack had appeared in the road and by the end of the week, an entire hill face had slipped away, taking the road with it.

An essential thoroughfare, the Parapara Rd opened in 1917, slashing the travel time between Raetihi and Whanganui in half.

Its closure affects all sorts of motorists from visiting tourists to travelling locals as well as freight truck drivers who must all now use alternate routes.

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Kennedy said there is a lot more work to be done to understand how the site of the slip is going to change over time.

"We will continue to monitor the site remotely until it is stable enough for ground crews to enter safely. The safety of road users and workers is paramount.

"Finding a safe solution for reinstating the road is a high priority, but given the scale and complexity of this slip it is going to take some time and a considerable effort."

The road will remain closed for a significant time and motorists are advised to plan for this.

The recommended detour route for motorists is via SH1 and SH49, which will add at least one hour to journeys.