The Transport Agency says the best long term solution for permanently re-opening the Manawatu Gorge road is benching the hillside where a massive landslip came down in August 2011.
Benching reduces the natural angle of the hill to make it more stable.
When falling rocks and debris were a problem at Arthurs Pass in the South Island, a rock shelter was built to protect vehicles on the road below .
But Manawatu and Taranaki state highways manager David McGonigal says a rock shelter would not work at the Manawatu Gorge, as the problem is worse.
He says the rock shelter is designed to protect the road from small rocks and debris, whereas in the gorge the hillside is unstable and slipping.
Mr McGonigal says the road also partly consists of bridges, and once the slip is cleared, damage to them might also be revealed.
The section of State Highway 3 has been closed since August 2011 by a landslip.
Drivers are using longer routes across the Saddle Road and the Pahiatua Track which are less suitable for heavy vehicles.
The Transport Agency says the gorge is expected to reopen in the middle of the year.