Rotorua was one of the coldest spots in the North Island overnight, reaching a low of -2.9C.
MetService meteorologist John Law said the city was "definitely in contention" for the title of coldest spot, particularly when looking at populated areas.
"We've got plenty of frosts around. [Rotorua] was one of the coldest in the North Island."
Between 5am and 6am Taupō also reached a low of -2.9C.
Rotorua swimming coach Henk Greupink said it was hard to get down to the pools for the 5.30am start in the cold.
"The gear bags are frozen, there is ice on the kickboards, but it's easy getting the swimmers in the pool because that was 27.5C."
He said as the water splashed over the edge of the pools it was turning to ice.
"It's just beautiful, so unique."
When Reporoa resident Deidre Orford woke at 7am "it was still cold enough not to want to get out of bed".
When her son Connor, 19, went to get the rubbish bins he found ice half-a-centimetre thick on top of a bucket on their property.
The Orford family covered their cars last night after yesterday's frost.
"Luckily it was not quite as cold this morning as yesterday. I was frozen out of my car yesterday morning and it took ages to get enough warm water to get in," Deidre Orford said.
The temperature on the Desert Rd reached a bone-chilling low of -5C overnight while Waiouru logged -3.6C.
Rotorua was colder than other populated areas in the Bay of Plenty, including Tauranga which logged 1.7C and Whakatāne which was sitting at 0.4C overnight.
John Law said Rotorua residents could expect the cool temperatures to stick around.
"If you go through the next few days, there might be a bit more cloud today but as a high pressure builds we've got more cold nights to come. Particularly on Friday night, Saturday morning," he said.
"If you're out first thing on Saturday morning do watch out for icy patches on the road."
Law said the temperature in Rotorua was logged at the airport so it could differ in different areas.
MetService lead meteorologist Mark Todd said the coldest areas this morning were in the central North Island high country, but a layer of cloud kept temperatures higher than expected in the South Island.
The forecast for Rotorua today is a high of 11C and low of 1C, cloud increasing in the evening and light winds.