Work is expected to begin early next month to get the Haast Pass reopened to two-lane traffic fulltime by the middle of this year.
The New Zealand Transport Agency has spent about $3.8 million to date managing the slip at Diana Falls on State Highway 6, which occurred during heavy rainfall on September 10. Since then, the road there has been cut to single-lane traffic from 8am to 7pm, and the route closed at night.
There had been delays in getting the work under way as NZTA finalised what was needed on site, completed the design, sourced material and appointed a contractor - which would happen in the next fortnight, senior network manager Mark Pinner said.
The first phase of the work will be building a fence draped up against the 30m vertical rock face that rises up from the road, with a large opening at the top of the rock face channelling any falling material against the rock face to prevent it from falling on the highway.
A temporary fence will then be built at the top of the slope to enable the bigger, up to 200-tonne rocks at the head of the slip to be broken into smaller, one-tonne rocks for removal.
Finally, a fence will be built about one-third of the way up the slip which has the dual role of catching any larger rocks that might move off the slope and to contain gravel expected to continue to wash down the rock face as the slope re-establishes its natural stability level.
''At this stage it is expected the works will begin within the next two to three weeks and be completed by the end of May. However, weather will be a significant factor in achieving this date,'' Mr Pinner said.
Additional traffic delays were likely throughout the day to complete the work and NZTA would provide as much warning as possible to ensure motorists could plan their travel through the area.
The cost of the work was not known until a contractor was appointed and the specific detail of the programme finalised.