Acting Senior Sergeant Mark Lucas, of Invercargill, said police had been concerned freezing conditions causing ice and black ice on roads would make driving hazardous and they urged drivers to be vigilant in winter road conditions.
Ice is likely again in some parts of Northern Southland this morning.
Emergency services were stretched around the district yesterday as black ice and fog resulted in three buses rolling within minutes of each other.
Sergeant Todd Hollebon, of Te Anau, said seven additional police staff were brought in and staff rostered off had been called in after a bus carrying 13 Korean tourists and a Korean driver to Milford Sound rolled on the Milford Rd, killing one woman and seriously injuring another four tourists.
The woman, yet to be identified, died at the scene, Sgt Hollebon said.
Two Southern Lakes Helicopters and the Otago Regional Rescue Helicopter attended the scene.
Pilot Graeme Gale said while flying conditions were fine, patches of fog at the scene forced him to land about 2km down the road.
Emergency services "did a splendid job" preparing the patients and the rescue effort was well co-ordinated in difficult, icy conditions and a remote location, he said.
Two passengers were flown to Southland Hospital in Invercargill and another three - including one with critical injuries - were flown by Mr Gale to Dunedin Hospital.
The rest of the party were taken to the Te Anau Medical Centre where they were treated for minor injuries.
Two patients were still being assessed last night in Dunedin Hospital but seemed to be "OK", a hospital spokesman said.
One patient was admitted to a general ward in Southland Hospital in a stable condition, a spokesman said.
Sgt Hollebon said the road was closed for most of the day because of the conditions and to allow the police serious crash unit and the commercial vehicle investigation unit to investigate the scene and to move the bus.
Severe frost was a factor in two more bus crashes in Northern Southland - both occurring within minutes of the Milford Rd crash.
St John Ambulance Southland District operations manager Pauline Buchanan said a van carrying six people rolled on a corner covered in black ice near The Key, west of Mossburn, at 8.15am.
One person was taken to Southland Hospital with moderate injuries, Mrs Buchanan said.
The remaining passengers, described by police as "shaken . . . [with] minor injuries", were taken to the Mossburn Fire Station.
The third serious bus crash occurred at 8.47am near Athol, on State Highway 6, when a bus carrying 18 Chinese tourists rolled.
Eight people were taken to Queenstown's Lakes District Hospital and the Queenstown Medical Centre with injuries, including a fractured cheek, concussion and minor cuts and bruises, Police Southern District communications manager Vivien Pullar said.
Inspector Olaf Jensen, of Invercargill, said two non-injury crashes about the same time were "possibly attributable to ice".
Although investigations were not complete, Insp Jensen said black ice and freezing fog were a common factor in all five accidents.
Senior Sergeant Bruce Terry, of Te Anau, urged drivers to take care on icy roads around the region.
"Police have been shocked by the speed drivers have been travelling at and at the manner of driving, given the very heavy frost," Snr Sgt Terry said.
The Milford Rd (State Highway 94) reopened about 5pm yesterday, with the New Zealand Transport Agency warning motorists to take extreme caution there as well as on the Te Anau-Mossburn Highway (State Highway 94) between Lumsden and Te Anau.
