However, in the valley suburbs, the temperatures could also have been sub-zero, he said.
"They are so-called ‘frost hollows’ that you sometimes get, where that cold air sinks down into those valleys."
It left many cars iced over and some streets with black ice.
Sergeant Matt Lee, of Dunedin, said four accidents were caused by icy roads yesterday morning.
The first was in Kaikorai Valley Rd, between Townleys Rd and Ensor Rd, where a ute slid off the road and into Kaikorai Stream about 7am, finishing upside down.
The 19-year-old driver had a lucky escape.
Fire and Emergency NZ Senior Station Officer Ant O’Neill said the ute was "fully submerged" in the frigid water, but the driver was not injured.
A grit truck was sent out to the area soon after.
SSO O’Neill urged drivers to slow down and drive to the conditions.
"And, also, if you see emergency vehicles at an incident, slow down and take care going through the scene.
"We’ve had multiple cars come though the scene today and they’ve nearly hit our fire truck and other responding vehicles."
Nearby, at the Southern Motorway’s Concord off-ramp, a car and truck collided after slipping on ice about 8.10am.
Moments later, while the drivers were exchanging details, two more cars collided behind the incident.
No-one was injured in either crash, and another grit truck was sent out soon after.
A fourth crash happened in Glen Rd, near Harwood St, where a van slid on ice and hit a power pole about 8.15am.
Sgt Lee said icy conditions would become more prevalent in the coming months, and he too urged drivers to take more care on frosty mornings.
"Drive to the conditions, and try to build in some more time because it will take you longer to get there.
"This is the time when you are most likely to encounter black ice on the roads, so be careful in the mornings."
The coldest place in the southern regions yesterday was Alexandra, which recorded -4.6°C.
Mr Law said there would be more cloud cover over the next few days, which would keep the frosts at bay, but the long-term forecast showed high-pressure systems with clear skies returning.
"I think we’re going to find our temperatures will be on the cold side, so we will find more frosts."
