Weather: Roads at risk, ferries cancelled as monster waves, cold snap strike

Ferries have been cancelled and road snow warnings issued as a gale-force southerly storms through the Cook Strait, whipping up monster waves and forcing a cold blast on the North Island.

Giant waves as much as six metres tall are swamping Wellington as the front moves through, causing the Interislander ferry to cancel several trips between Wellington and Picton on Wednesday and Thursday.

Interislander executive general manager Walter Rushbrook told RNZ the company regretted the inconvenience however the top priority had to be safety.

A NIWA graphic shows the force of the wind, with much of the country struck by powerful gusts.

"Wellington and the eastern South Island will certainly 'raise the bar' on Wednesday-Wednesday night!" it says.

"Blustery southerlies will affect the remainder of the North Island on Thursday."

This wind has churned up powerful swells off the coast of New Zealand.

"A persistent strong southerly flow is forecast to generate sizable swells along the eastern coasts of Aotearoa on Wednesday," MetService warns.

"Long period 4-5m swells are likely around coasts open to the south."

And the cold wind has also forced temperatures down, with both Taupō and Blenheim sub-zero this morning.

"Cold southerlies cover the country this working week with temperatures expected to remain on the wintry side," MetService says on Facebook.

"Here are your observed temperatures across the main centres this morning. Nothing into the double digits and a few stations still in the negatives."

In the central North Island, large snowfalls are forecast and a heavy snow warning has been issued for the Tararua Ranges and the central North Island.

Several road snow warnings have been issued for the Napier-Taupo Road (SH5), Desert Road (SH1), Remutaka Hill Road (SH2) and Lewis Pass (SH7).

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