New Zealand sweated under its third-warmest temperatures on record last year, according to Niwa's annual climate summary.
The country also suffered one of its worst droughts on record.
Niwa's principal meteorology scientist Dr Mike Revell said the national average temperature last year was 13.4C, 0.8C above average and the third warmest on record since 1909.
The hottest year was 1998, followed by 1999, he said.
"I think it indicates that there's been a significant upward trend in the temperatures.
"There's no guarantee that next year will be warmer again, but we are expecting temperatures to continue to increase.
"The general drift of temperatures over the past 100 years...I think you could probably fairly safely associate that with global warming."
Above average northerly winds last year, bringing warm air from the tropics, contributed to the warmer average temperature across the country, Dr Revell said.
"The winter was the warmest we've ever had, so although the year itself was the third warmest, the actual winter was the warmest that we've had since 1909," he said.
The southerlies which hit Wellington in June were some of the worst on record, while the big drought which crippled farmers for the first three months of the year was alleviated in April by heavy rain, Dr Revell said.
"That was one of the worst droughts we've experienced for a long time."
Of the six main centres, Auckland was the warmest last year, Tauranga was the sunniest, Christchurch was the driest, Wellington was the wettest, and Dunedin was the coldest and cloudiest.
Last year was the warmest year on record for Tauranga, and the third warmest year on record for Dunedin.
Over 2013, eleven tornadoes and four waterspouts occurred, from as far south as mid-Canterbury to as far north as southern Northland.
While they didn't cause any injuries or deaths, they were considerable damage caused to individual properties and vegetation.
2013 was the driest year on record for Dargaville, Toenepi (near Morrinsville), Taupo and Turangi, with these locations each recording between 67 per cent and 72 per cent of normal annual rainfall.
Above normal rainfall was recorded in parts of the eastern South Island including Marlborough, North Canterbury and North Otago. Ranfurly, Alexandra and Lumsden all had near-record high annual rainfall totals.
• The highest air temperature was 35.1C, recorded at Clyde on January 5, and at Gisborne on both January 9 and 10.
• The lowest air temperature was -12.1C recorded at Lake Tekapo on June 28.
• The highest confirmed wind gust was 202 km/h, recorded at Mt Kaukau (Wellington) on June 20.
• The top daily rainfall was 346mm, recorded at Mt Cook Village on January 9.
• The wettest location was Cropp River (West Coast) with 10870mm of rain falling last year.
• The driest location was Lauder, Central Otago, with a mere 453mm of rainfall recorded.
• Whakatane was the sunniest location, recording 2792 sunshine hours.
• Auckland: mean temperature 15.9C (0.5C above average), 1145mm of rainfall, 2199 sunshine hours
• Tauranga: mean temperature 15.8C (0.9C above average), 1052mm of rainfall, 2515 sunshine hours
• Hamilton: mean temperature 14.2C (0.6C above average), 1086mm of rainfall, 2114 sunshine hours
• Wellington: mean temperature 13.7C (0.8C above average), 1409mm of rainfall, 2108 sunshine hours
• Christchurch: mean temperature 12.2C (0.6C above average), 683mm of rainfall, 2024 sunshine hours
• Dunedin: mean temperature 11.8C (0.7C above average), 775mm of rainfall, 1817 sunshine hours