New Zealand is now experiencing the worst of the fire season, a top officer says, with a dangerous combination of high temperatures and low relative humidity creating "trigger point" conditions across eastern hot-spots.

A more extreme southwesterly summer regime has kept much of the North Island's East Coast and vast stretches of Canterbury dried out - and with plenty of fuel to burn.

As the summer progressed, rain-deprived eastern landscapes had transformed from vibrant green to the colour of golden straw - and cured grass at this stage heightened the potential for a fire to ignite, spread and create flame fronts more than two metres high.

Strong northwesterly winds had also been a major factor in pushing fire severity and danger levels to extreme, especially for the East Coast, where nearly 40 people were this week evacuated during a major wildfire near Hastings.

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Further south, on the edge of Christchurch, more than a dozen helicopters, a plane and 100 firefighters were sent in to battle two massive wildfires, which had torn across more than 700ha since erupting on Monday evening.

The other major blazes that rural fire crews had faced this summer - including major events at Mahanga, Gisborne, Marlborough and Whitianga - also pointed to an East Coast primed for fire.

Outdoor fires are now banned across Northland, Coromandel, the Hauraki Gulf, Gisborne, Hawkes Bay, Marlborough Christchurch, Banks Peninsula and Central Otago.

"We are probably in about the worst of it at the moment," said the National Rural Fire Authority's rural fire manager, John Rasmussen.

"While February is traditionally our busiest month, because it's drier, the wind is not usually bad as it has been this year."

Rasmussen said the authority was concerned that a dangerous cross-over effect in the weather - where temperatures, reaching into the mid-30s in some places, became higher than relative humidity, which was dropping as low as 29 per cent - was creating a "trigger point" for wildfires.

"For us, that's when conditions become critical - and it's when it gets difficult if a fire does start."

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The extreme fire danger also meant fire crews had to be especially careful, particularly when battling blazes uphill and down-wind.

"In some cases, it means they can't actually get in there and do a hell of a lot until something changes: whether it's the fuel, the weather or the topography."

While temperatures are predicted to remain near or above average in eastern areas until April, Rasmussen expected the easing of winds into autumn would help lower the risk.