The South Island could be due for more extreme rainfall events later this year, according to the National Institute of Water and Atmospheric Research (Niwa).

And the balmy days of summer are still a long way off, with the whole country expecting average or colder than normal temperatures between October and December, according to the latest seasonal outlook from climate scientist Neva Fedaeff and meteorologist Ben Noll at the National Institute of Water and Atmospheric Research (Niwa).

Over September a climate driver called Sudden Stratospheric Warning brought chilly weather to New Zealand as well as to Argentina, Chile and south Australia, Niwa meteorologist Ben Noll said.

September was the first time in 32 months that New Zealand had had below-average temperatures, Noll said. And it's a sign of things to come - there are unlikely to be any above-average temperatures in the next three months.

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It was the fourth-coldest September since 2000 and sea temperatures had all been below average last month for the first time since January 2017, Noll said.

The first half of October was looking reasonably chilly but some warmth was coming mid-month, particularly in the east and the upper North Island.

The SSW's influence continued into October but would fade moving through spring, with a phenomenon known as the Indian Ocean Dipole becoming the primary climate driver for the next three months, Noll said.

The dipole - lower than normal sea temperatures around Indonesia and higher than normal temperatures in the Arabian Sea - was "strongly positive", Noll said. Last year a similar dipole had driven extreme rainfall events that saw records broken in parts of the South Island.

In November wet weather was likely for the central and lower parts of both islands, including Raglan, Hamilton, Rotorua down to Ohakune and northern Taranaki.

South Island's hydro lakes will get a much-needed boost after dropping over September.

The wet trend would continue into December with more rainfall than average in the South Island and the west and upper north of the North Island.

It would be "good news for farmers, perhaps not so good for beachgoers", he said.

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There would be an increase in soil moisture, "that could tip the scales".

There was little chance of above average temperatures, in fact western and northern parts of the South Island could see normal or below normal temperatures over the next three months due to the various climate drivers.

Niwa will publish its tropical cyclone outlook next Friday.

NIWA'S SEASONAL FORECAST
OCTOBER - it's chilly however, remains relatively dry in east and northern North Island.
NOVEMBER - the rain arrives, centring around Central North Island and
DECEMBER - wetter than normal in South Island and west and upper north of North Island