Last year also marked a new record for the warmest minimum temperatures, previously held by 2016.
In Hastings the mean temperature was 14.9C, a whopping 1.8C above normal and the warmest since records began in 1965.
Wairoa's mean temperature hit 15.3C, 1C above normal, and the town's third highest since records began in 1964.
The annual mean temperature in Napier at Niwa's climate station was 15.2C. It ranks as the 6th warmest year on record in the town since records began in 1870.
The warmest year on record in Napier was 1916 when the annual mean temperature was 15.8C.
Niwa principal scientist Chris Brandolino said 2018 was part of a larger warming trend, with four of our past six years in the top five hottest years on record.
Key stats included:
• January was New Zealand's warmest month - ever, at 3.1C above average.
• Three ex-tropical cyclones struck New Zealand in 2018: Fehi, Gita and Hola.
• The hottest recorded temperature was 38.7C in Alexandra on January 30. The coldest was -10.4C at Mt Cook on June 3.
• The wettest part of the country was Milford Sound, recording 6818mm. The driest was Clyde with 526mm of rain.
Brandolino said January was the warmest month of any year on record, at 3.1C above average.
He said there were three main drivers of the warm temperatures.
The first was ocean temperatures, with New Zealand experiencing a "marine heat wave" in the first few months of the year. Some parts of the sea recorded temperatures 2C above average.
"New Zealand is an island nation, so where ocean temperatures go, they also go on land."
New Zealand also experienced more subtropical airflow in 2018, with many lows to the west and highs to the east driving northeast winds.
The increase in greenhouse gases, surpassing 400 parts per million, was the third factor, providing a "long-term tailwind to temperatures".
Wake-up call for action on climate change
Climate scientist Dr Jim Salinger had predicted last week 2018 would be our hottest year on record, but was happy with Niwa's conclusion.
"We used slightly different calculations, but we are all consistent in that we are in the hottest period on record."
Niwa also found 2018 had the hottest minimum, or overnight, temperatures on record.
Salinger said this was a "wake-up call" for politicians to take action on climate change.
"It is very concerning, and politicians are not yet doing anything about it.
"We recognised this was happening back in 1980, now it is starting to almost get too late.
"The young are calling for it, so let's get the Zero Carbon Bill in and with bipartisan support. The evidence is out there.
"As we see further temperature rises we are going to see a lot of extinctions, people living around the coasts and in small island states will be affected by rising seas, and our oceans will become more acidic."
Already in New Zealand we were experiencing dramatic impacts, particularly in the Southern Alps.
Last summer's "unprecedented" heatwave covered an area the size of the Indian subcontinent - 4 million square kilometres. It led to the largest annual ice melt from our Southern Alps glaciers when 3.8cu km, or 9 per cent of permanent snow, went down our rivers.
This brought the total down to 37.5cu km, compared with 60cu km in 1962, a 38 per cent reduction.
Last summer's heat saw very early Sauvignon Blanc wine-grape maturation in Marlborough, and major species disruption occurred in our marine ecosystems with snapper caught in Fiordland.
