The NZ Transport Agency is assessing a large slip on State Highway 4, which is set to keep the Whanganui to Raetihi route closed until late Friday, and possibly longer.

The slip north of Fields Track at the Hapokopoko Bluffs, dumped approximately 500m3 of material onto the road.

Contractors and geotechnical engineers who are on-site have identified a large deposit of loose material at the top of the bluffs, which is unstable.

Regional Transport System Manager Ross I'Anson says a helicopter will be on-site Friday to start a sluicing operation to dislodge approximately 200m3 sitting up above the road.

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"This slip will keep the road closed for road users from Whanganui to Raetihi until the end of Friday, and potentially longer until the situation is resolved. However local residents may be able to access the route up to Fields Track from late tomorrow (Friday) afternoon."

Fields Track is only suitable for light vehicles and the Transport Agency recommends alternative routes for heavy vehicles.

Mr I'Anson says that decision will be reviewed Friday afternoon at 4pm after further assessments are made on the condition of the slip site and taking into account the forecast weather event this weekend.