One lane was closed yesterday afternoon after contractors discovered a section of culvert underneath State Highway 1 had been washed out by a stream.
Rivers and streams in the South are running high because of heavy rain and snowmelt.
New Zealand Transport Agency (NZTA) Otago-Southland operations manager Roger Bailey said the problem was discovered about lunchtime.
The 900mm culvert had been washed out, and the road surface had formed a lip over the hole, Mr Bailey said.
The road was closed at 5pm, and Mr Bailey was unsure how long it would take to repair the damage.
''We'll be starting work as soon as we can.''
Contractors planned to block the nearby stream and pump water across the highway to allow them to work in dry conditions inside the culvert.
A detour is in place along West Otago Rd at Waipahi and Young Rd at Arthurton. Motorists are advised to allow an extra 10 minutes' driving time.
The detour for heavy vehicles is State Highway 93 between Clinton and Mataura.
