The all-night effort was to help motorists facing a chillier-than-usual morning commute.
Temperatures as low as -3.9degC at Dunedin International Airport led to frosty streets, and several minor motor vehicle accidents were reported around the city.
Three Mile Hill Rd, between Dunedin and Mosgiel, was closed about 1.30am because of black ice, and did not reopen until 12.15pm.
Fulton Hogan regional manager Richard Fulton - whose company has the contract to grit council roads - said two of the company's grit trucks were rolled out about 10.30pm, at the request of police, who were concerned about the incre- asingly treacherous condit- ions, he said.
Dunedin City Council roading maintenance engineer Peter Standring said the frost was the heaviest, and most widespread, this winter, and Fulton Hogan contractors had spread more grit than on any other night since last winter.
"We put just over 209 tonnes [of grit] down last night. That's one hell of a lot of grit," he said.
Forty tonnes of grit were spread across city roads on Monday morning, when black ice around Waldronville, Green Island, Kaikorai Valley and the hill suburbs led to 15 crashes.
Yesterday's heavy frost had been forecast and contractors had responded well, meaning few problems were reported, Mr Standring said.
"These guys are making some difficult calls in the middle of the night.
"To put 200 [tonnes] down was a good call," he said.
Downer EDI Works area supervisor Trevor Cunningham - whose company has the contract to grit state highways around Dunedin - said a truck spraying the de-icing agent calcium magnesium acetate (CMA) was operating from 6pm on Tuesday to 6am yesterday, and another five grit trucks began work at 5am yesterday.
Together, the six trucks spread about 28,000 litres of CMA and 30 tonnes of grit on the city's one-way streets, parts of State Highway 1 north and south of Dunedin, State Highway 86 to Dunedin airport, State Highway 87 from Mosgiel to Riccarton Rd and State Highway 88 to Port Chalmers.
Mr Fulton said two of his company's trucks operated through the night, liaising with police, until joined by four other Fulton Hogan trucks about 4.30am.
The early-morning gritting focused on keeping main arterial roads, and known danger spots passable for commuters, he said.
Ice had been particularly noticeable around Mosgiel, but had descended like "a blanket" across the city, he said.
