The extreme fire risk that contributed to this week's devastating Tasman infernos could more than double by the end of the century according to climate models.

Firefighters have been battling the blazes ravaging the Tasman district south of Nelson City since Tuesday afternoon, and are expected to face another 48 hours of "very hard firefighting" after the fire area expanded Thursday night.

It grew to 2000ha last night, from 1600ha.

Scion rural fire scientist Grant Pearce said the Nelson area averaged about nine to 10 days a year of very high or extreme fire danger, with 16 or 17 in a particularly bad year.

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Already this year the region had already experienced seven days of very high and eight days of extreme danger.

Fire danger came from a combination of weather factors, including high temperatures, low humidity and wind speed.

Fire managers had reported current fire danger levels were the highest they'd seen in almost 20 years, and Niwa's soil moisture maps are showing significant soil moisture deficits across the region, Pearce said.

"The very hot, dry and windy weather over the past month has had a major effect on the drying of forest fuels in particular, including slash and prunings, understorey scrub vegetation, pine needle litter on the ground and organic material in the soil, which all contribute to the amount of vegetation fuel available to burn.

"These dry, elevated fuel loads contribute to easy fire ignition and spread, and to high fire intensities including crown fires that are very difficult, if not impossible to control, especially in steeper terrain often favoured for forestry plantings.

"Surrounding grass fuels are also dry, but the lower fuel loads mean that fires are usually easier to control once they spread out into these grass areas."

The extreme conditions that preceded this week's Tasman fires are expected to more than double by in frequency by the end of the century with climate change. Photo / Aimee Jules
The extreme conditions that preceded this week's Tasman fires are expected to more than double by in frequency by the end of the century with climate change. Photo / Aimee Jules

These factors were all on course to increase in intensity with climate change, Pearce said.

"Modelling of potential future changes in fire danger indicates that the number of severe fire weather days is likely to increase in many parts of the country," Pearce said.

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"This includes the Nelson region, where the number of days of very high and extreme forest fire danger could increase to more like 12-13 days per year on average, and maybe 20-25 days in the worst years, with climate change."

Modelling by Scion last year found that almost all areas of the country would see an increase in fire danger, with the average fire season length increasing by about 70 per cent up to 2040, and by about 80 per cent up to 2090.

Although the most fire-prone regions of Gisborne, Marlborough and Canterbury would remain the most at risk, the relative increase in risk was highest in Wellington and coastal Otago, where it could double and triple to 30 days and 20 days per season, respectively.

The Nelson fire seen after a flare up near Wakefield. Photo / Aimee Jules
The Nelson fire seen after a flare up near Wakefield. Photo / Aimee Jules

Pearce said measures to counter the increasing fire risk included fire breaks being factored into forestry plans, dams built nearby for extinguishing fires quickly, and increasing the availability of firefighting resources in high risk areas.

It was also important for planners to factor fire risk into where houses and other properties were built.

"This fire was in a more rural area, but fires like the Port Hills fire showed what can happen when it backs onto a heavily urban area, so it is a key consideration for future planning about how to mitigate fires in those areas," Pearce said.

NIWA meteorologist Ben Noll said December through January had been the third warmest start to summer on record.

The warm pattern was a result of very warm sea-surface temperatures both in New Zealand coastal waters and in the Tasman Sea, as well as a lack of cooler southerly winds.

In particular Richmond, in the Tasman District, had two consecutive days at the end of January reaching 33.8C.

"January came with abundant sunshine and it was the sunniest month on record for any location in the South Island, with Richmond recording 355 hours," Noll said.

The first flames caused by a spark from agricultural machinery on a Pigeon Valley farm. Photo / Joel Scott
The first flames caused by a spark from agricultural machinery on a Pigeon Valley farm. Photo / Joel Scott

Along with the warm it has been very dry, with Richmond going 22 days with less than 1mm of rain as of February 6.

Overall just 84mm of rain was recorded from December to February 5. The normal summer amount to February 28 is 224mm.

The New Zealand Drought Index was reporting "extremely dry" conditions in the Tasman-Nelson area, with long-term dryness beginning in November 2018. Similar drought conditions were also reached last summer (2017-18).

According to the Intergovernmental Panel on Climate Change (IPCC), heatwaves and fires were "virtually certain" to increase in intensity and frequency," Noll said.

"Droughts are very likely to become more frequent and intense."

The Nelson region has a history of significant forest fires, with major ones occurring in February 1981 at Hira, on the outskirts of Nelson city, which burned 1972 ha; in October 1997 at Harakeke, in the Moutere Hills (535 ha); in December 2004, in Irvines Forest (200 ha); and in November 2009, near Tadmor (600 ha).