The second warmest January was in 1956 when the average was 18.8C.
Salinger had downloaded the climate data from Niwa's seven stations which were used to calculate New Zealand's average temperature.
The stations are located at Auckland Airport, Kelburn in Wellington, Masterton Airport, Hokitika Airport, Nelson, Lincoln near Christchurch and Musselburgh, Dunedin.
With 10 days left in January the average temperature was unlikely to drop given there were no "cool southerlies in the weather projections".
"The door to the southern oceans is shut.
"For the next period in the end of the month there is nothing cold coming from the south, the ocean temperature in the South Island is two degrees warmer than average," Salinger said.
There was a possibility also that January could take over February as the hottest month in recorded history.
"It could increase further, what's interesting is if it will exceed the warmest month which was February 1998 at 19.6 degrees."
Behind that was February 2016 at 19.5C.
"Another half degree would make it the hottest month ever, right now it [January] is the third warmest month ever."
In general temperatures were 0.9C higher than 1909 which showed there had been "a certain amount of global warming".
"It's telling me the thing that has caused the December heatwave and the marine heatwave is still going.
"The highs are crossing the Tasman Sea and stopping east of the country, the eastern flow is stopping any stormy stuff coming across the Tasman".
The country was in the middle of a marine heatwave which was marked to continue to February.
"It will be interesting to see what the impacts of the marine heatwave will have on marine life, especially shellfish and kelp.
"Once would expect with global warming to have it [the record] exceeded in the future, at the moment we are in very favourable weather patterns".
February 2018 like January could be the warmest on record.
If that was the case then it was possible that this summer would be the hottest in recorded history, he said.
MetService meteorologist Sarah Haddon said January had seen Invercargill and Dunedin break their hottest temperature records which was a good indication there had been some "unseasonably hot weather this month".
Dunedin Airport hit 34.8C on January 16 and Invercargill Airport hit 32.3C on January 14.
The heat was also being felt in Australia with areas in Victoria, New South Wales and Western Australia over the weekend hitting 40C as the country suffered through a heatwave which was now entering day five.
