The National Institute of Water and Atmospheric Research (NIWA) has declared the official arrival of El Niño.

The weather pattern - which usually causes dryness in the east and more rain in the west - is likely to continue over summer, NIWA said.

It said the country will see dramatic temperature swings over the next three months, with periods of unseasonable warmth followed by sharp cold southerlies.

It would be a "temperature rollercoaster", NIWA meteorologist Ben Noll said.

"In terms of our temperatures, we could be at near 30 degrees one day and then maybe 15 degrees the next day.

"We're talking about some really dramatic swings."

The north and east of the North Island could experience below-normal rainfall while the west of the South Island was most likely to see above-normal rainfall.

"What we saw last week for Southland, interior Otago, the west of the South Island or that heavy rain event in the flooding, that's a theme that may be reoccurring for the next three months," NIWA principal scientist Chris Brandolino said.

There is going to be a variety of impacts across the country as El Niño beds in," Noll said.

There would also be stronger than usual winds, with the pressure pattern delivering more westerly winds that would likely lead to some prolonged dry spells to the north and east of both islands, NIWA said.

It said strong fronts will occasionally impact the western and lower South Island, delivering heavy rainfall and a risk of flooding.

Rainfall is most likely to be below normal in the north and east of the North Island and above normal in the west of the South Island.

The fire risk will be higher than last year for some areas, but marine heatwaves were expected to be less persistent and widespread.

There was around a 100 percent chance of El Niño continuing during October-December and over 95 percent chance it will persist through summer, NIWA said.

"The wildfire season this year is certainly going to be different from probably the last couple," Fire and Emergency national wildfire manager Tim Mitchell said.

He said he thinks there will be a see-sawing of fire danger where we go through high and low periods.

"But as we get closer to Christmas and we don't see that rainfall keeping up we think that we're probably going to go into a probably a more drying period and that high fire danger risk."

Now was the time to prepare, he said, clearing vegetation, setting up water supplies, cleaning gutters and forming a plan.