A major slip on State Highway 3 means the road south of Mokau will remain closed until Thursday midday at the earliest.

The NZ Transport Agency warned motorists wanting to travel between Taranaki and the Waikato of the closure tonight.

The blockage followed a major slip blocking the road from Sunday, triggered by a large amount of rain over the weekend.

"Getting this road open safely is a top priority for us,' said NZTA spokeswoman Karen Boyt.


"We know that it is the main route between Taranaki and the Waikato and the road closure and long detour are frustrating and inconvenient for many people.

"The advice from our geotechnical engineers is there is still currently risk of further major rock fall at the site and it is not safe for either contractors or motorists to be on or near the site."

Helicopters with monsoon buckets of fresh water have been washing down the remaining loose rock since yesterday.

Abseilers were scaling the rock face today to check remaining rocks and knock any off loose ones.

Once geotechnical engineers have confirmed that the cliff is stabilised contractors will begin to clear the rocks, allowing the road to open to one lane.

"Once the rock face is confirmed as safe, our contractors will then be allowed in to remove the additional rocks ... clear the road to make it safe for motorists, and allow us to reopen the road," Boyt said.

"We estimate that over 200 cubic metres of rocks and debris will need to be cleared, and contractors will be working on site for the best part of a day.

After the road reopened, NZTA would be investigating a more permanent engineering solution to replace the wire mesh and ensure long-term protection of the slope, Boyt said.

The Transport Agency would provide another update on Wednesday afternoon with more detail on the timing for clearing the rock and debris, and when the road would re-open.

Until SH3 re-opened, motorists were advised to use an alternative route via SH1 or SH4 via Whanganui, which will add approximately three and a half hours to the journey.

NZTA warned other local road routes via Ohura and SH43 were not suitable detours for SH3 traffic due to how narrow and windy they were.