A cyclone swirling in the Pacific north of Aotearoa has been upgraded to severe.

WeatherWatch this morning said Cyclone Winston had ramped up to Category 4, which would increase the chances of more dangerous beach conditions for parts of the North Island.

The cyclone posed no direct threat to New Zealand, as it was likely to swing back north after tracking south.

"However this swing back north in a few days may impact Tonga -- and help set up very humid airflow down into New Zealand later next week," head weather analyst Philip Duncan said.

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Cyclone Winston has sustained winds of 175km/h and is still strengthening. Winds were expected to get to 185km/h today, with gusts well over 200km/h.

A second tropical cyclone, Tatiana, remains at Category 1 in the Coral Sea. It was likely to lose its cyclone status this weekend.