The year was the warmest on record since Niwa started using its seven-station temperature series in 1909, with the nationwide average temperature for the year at 13.4C.
Chris Brandolino, principal scientist and forecaster at Niwa, said ocean temperatures were unusually warm throughout NZ, which was impacting on air temperatures up north.
He said the warmer northern location and proximity to the water were responsible for the region's heat.
It has been the warmest year on record for all of New Zealand, because of the ocean temperatures and warm northerlies from the tropics and sub-tropics, he said.
"The climate has changed over the last 100 years with increasing greenhouse gases continuing the warming trends.
In 2015, Whangarei came out top of the country with the highest annual average temperature at 16.1C, with the title again going to Whangarei in 2014 with 16.1C.
In 2013 it was Dargaville with 16.5C, followed by Whangarei with 16.3C, while in 2012 Whangarei had the highest at 15.8C.
Temperatures in Whangarei were set to simmer in the mid-20s this week before soaring to 28C today, tomorrow and 29C on Sunday.
The official Whangarei temperature is collected at the airport and the thermometer would have reached up to 4C higher inland.
Niwa installed a climate station at the site of the Kioreroa Rd Water Treatment Plant in April 2015, however, that data is not yet included in climate summaries.
The highest annual total rainfall (11921mm) in New Zealand in 2016 was recorded at the Cropp River station, near Hokitika in the South Island.
Kerikeri recorded 1928mm, Kaikohe 1885mm, Kaitaia 1399mm, Whangarei 1236mm, Dargaville 985mm and Cape Reinga 834mm.
Meanwhile, the top sunshine hours in Northland were recorded in Kaitaia (2377), compared with the 2840 in Richmond, south of Nelson.
