The Manawatu Gorge will be closed for at least five more months.

The New Zealand Transport Agency announced today that the main route between Manawatu and Hawke's Bay would stay shut because of the amount of slip debris that needs securing and clearing.

Palmerston North state highways manager David McGonigal said although crews were making outstanding progress, there was still a lot of work to be done.

"We believe this is the largest slip ever to occur in New Zealand. We certainly can't find anything comparable ... It is a very complex slip and the challenge comes from the size of it and the environment we are having to work in. We are up against mother nature, this is what she has thrown at us."

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Mr McGonigal said crews are working seven days a week.

"The volume of work we have to get through is huge and we realise it will beat least the end of June before we can reopen the Manawatu Gorge.

"However, this is weather-dependent. Summer hasn't been kind to anyone and we cannot work up there when it is wet as everything just turns to mud and the machines slide around which is dangerous."

He also said there were a number of unknowns but the agency would continue to provide regular updates.

"We have no idea what state the bridges are in under the slip and we won't be able to assess the extent of the damage they have suffered until the debris is completely cleared.

"The crews are unearthing a lot of boulders and the biggest one we have uncovered so far, is approximately 30 tonnes. Along with the fractured rock and vegetation, it is a big job.

"The key thing is we are going as fast as we can and while we are looking at opportunities all the time to speed things up, we can only have so many diggers working at once. I will be very pleased when this road reopens."