Stabilising the slip on State Highway 6 between Lake Hawea and Haast, south of Makarora, will cost between $300,000 and $400,000, but possibly more.
New Zealand Transport Agency Central Otago area manager John Jarvis told the Otago Daily Times this week the costs were "very approximate" at this stage and could rise if it was found bolts needed to be drilled into large rocks above the road.
"We might have to rock-bolt some of the features to keep them up there."
Those features include big slabs of rock with cracks that Mr Jarvis said "at the moment are holding in there OK".
Rock-bolting work could add "tens of thousands of dollars" to the final bill, he said.
The slip, which came down more than three weeks ago, is still blocking one lane and causing the road to be closed some nights for safety reasons.
Mr Jarvis said a rock bund constructed beneath the slip to protect traffic had filled up in the past few days with rock brought down by heavy rain.
"We just can't control the situation in the dark."
Mr Jarvis said the land that slipped was not much different from other "active landslides" the NZTA monitored along that stretch of highway above Lake Wanaka.
"Over the years we've had lots of slips along there. We just haven't had many in the last few years."
It will be at least two or three weeks before both lanes are reopened.
