State of emergency as swells of up to 10m to batter NZ's east

Waves off New Zealand's East Coast could reach up to 10 metres in the coming days as a "rapidly rising" swell approaches from the south of the country, leading several Cook Strait ferry services to be cancelled.

On Monday afternoon, Wellington City Mayor Andrew Little declared a State of Emergency for Wellington’s Southern and Eastern wards to support the response to the heavy swell warning for Wellington’s south coast.

Wellington Regional Emergency Management said the swells tomorrow could be among the largest to hit the region in recent years.

"If your property has been affected by previous swell events, now is the time to prepare," it said in a statement, urging residents to move items from low-lying areas, prepare a grab bag and make a plan.

"The waves are expected to arrive from early on Tuesday morning, with the largest waves expected from 10am-1.30pm."

Heavy swell warnings have been issued for the Wellington and Wairarapa coasts, with dangerous conditions expected to peak on Tuesday. Strong wind watches were issued for coastal Wairarapa and coastal Clutha and Dunedin.

Wellington's coastline from Cape Terawhiti to Turakirae Head faces combined waves of 7-8m around midday Tuesday, while the Wairarapa coast from Turakirae Head to Mataikona is forecast to see even larger combined waves of 9-10m late Tuesday morning.

A Wellington City Council spokesperson said an Emergency Assistance Centre has been set up at Wellington South Baptist church, 284 The Parade, to be open from 9pm on Monday "for those who don’t have a place to go tonight".

"We recommend that residents of South Coast waterfront properties and those who are concerned about flooding stay somewhere else tonight," the spokesperson said.

"Residents of the South Coast can expect extensive road closures on Tuesday. We require those in affected areas including pets be out of properties no later than 9am on Tuesday."

Wellington Civil Defence Controller Phil Becker said the size of the forecast waves meant there is a real risk to the safety of residents on the coast and properties may be inundated by seawater.

The Wairarapa coast and Wellington Harbour are also expected to be hit hard by the swells. Large waves are expected in Wellington Harbour including Petone and Eastbourne. However, these impacts are not expected to be as significant.

Both major Cook Strait ferry operators have cancelled multiple sailings, with Bluebridge cutting its 8.30pm Monday Wellington-to-Picton service and three Tuesday morning departures, while Interislander scrapped all of its Tuesday sailings.

The companies said affected customers were being contacted and moved onto the next available services.

MetService said combined wave heights in eastern offshore waters could reach or exceed the height of the Ohakune Carrot (7.5m).

"Hazardous conditions mainly for offshore vessels operating in exposed eastern waters."

MetService meteorologist Devlin Lynden said the swell is being driven by a deep low pressure system sitting to the south of the country, generating very strong southwesterly winds.

"There's a deep troughing to the south of the country — a deep low pressure system — which is driving very strong southwesterly winds in the area," he told 1News.

Two key factors were combining to produce the dangerous conditions: wind speed and fetch — the distance over which a swell can build. In this case, the southwesterly swell has had virtually the entire Southern Ocean to grow across.

"The combination of large fetch and strong winds is what's creating these significant swells," he said.

The most affected areas would include offshore waters and the Chatham Islands, as well as coastal stretches along the southern Kaikōura coast, parts of Banks Peninsula, and the Catlins coastline south of Dunedin — all expected to bear the brunt of the swell over the next 24 to 48 hours.

Lynden said what makes this event particularly notable is how fast conditions will deteriorate.

"We are talking about rapidly rising, which is not a wording we use commonly, and it really reflects how quickly the swells are likely to rise in the affected areas."

Recreational boaties were urged to exercise extreme caution, particularly around harbour entrances and bar crossings, where large swells significantly increase risk.