With just three weeks until the end of the year, 2016 is set to break the previous hottest record set in 1998, Xinhua news agency cited the National Institute of Water and Atmospheric Research as saying.

Temperatures for the rest of the year would drop markedly to more than one degree centigrade below normal for the next three weeks to avoid the record, but that was unlikely, said NIWA forecasters.

The national temperature observations dated back to 1909 and the mean temperature for the first 11 months was running at 0.94 degrees centigrade above the 1981-2010 average.

The first seven months of the year were all warmer than normal with only August below normal this year.

NIWA principal scientist climate Brett Mullan said the figures were in line with global average temperatures and confirmed the longer-term trends of climate change.

"What we are seeing is the stark reality of global warming," Mullan said.

The exceptionally warm conditions for the first six months were a consequence of two main factors -- the long-term regional warming trend due to greenhouse gas increase in the atmosphere, plus local natural variability adding extra warming.

Globally, the World Meteorological Organisation said 2016 is set to be the hottest year on record by a significant margin with temperatures 1.2 degrees centigrade above pre-industrial times.

A new high will be set for the planet for the third year in a row.

The level of carbon dioxide in the atmosphere also broke records this year, with May seeing the highest monthly value yet at 407.7 parts per million at Mauna Loa in Hawaii.

The opposition Green Party said the New Zealand government's lack of leadership had been a disaster for the climate, with the nation's emissions up 19 per cent since 2008.

"New Zealand is currently emitting more than seven times our fair share of greenhouse gases per person," Green Party co-leader James Shaw said.