The operation to clear the Manawatu Gorge slip has so far cost the Transport Agency more than $11 million - and the final bill for the road repair will be much higher.
The section of State Highway 3 has been closed for nine months since 370,000 cubic metres of rock and soil slipped down the hill.
The landslide was finally cleared last week and the agency is rebuilding two destroyed bridges as it works to have the road open to one lane of traffic by the end of the month.
Transport Agency spokesperson David McGonigal says the cost to date of the slip operation and maintaining alternative routes has been substantial.
He says the final cost, once the new bridges are factored in, could be $19 - $20 million.
Members of the public are able to see the progress in an open day at the Ashhurst end of the Manawatu Gorge from 10am - 3pm on Sunday.