The first six months of 2016 were the hottest start to a year that's ever been recorded in New Zealand, scientists say.

Temperatures in the South Pacific nation were 1.4 degrees Celsius (2.5 Fahrenheit) above the long-term average for the first half of the year, according to the government-funded National Institute of Water and Atmospheric Research. That's the highest since record-keeping began more than a century ago, and significantly higher than the previous record of 1.1 Celsius above average, reached in 1938 and again in 1999.

Chris Brandolino, a scientist at the research agency, said three factors contributed: unusually warm ocean temperatures, warm winds from the north, and a background of global warming.

He says the agency is predicting warmer temperatures will continue for at least another three months.