Wellington: A wild storm battered New Zealand on Thursday, sparking a state of emergency in flooded South Island towns.

As the former Tropical Cyclone Fehi slammed most of the country, heavy rain flooded the southern city of Dunedin and west coast town of Buller, forcing authorities to declare a state of emergency and ask people to avoid travelling by road.

In Nelson, at the northern tip of the south island, was hit by chaotic weather, with people evacuated from their homes and numerous roads closed. The airport was closed due to flooding and around 50 people were evacuated from areas by the coast.

Nelson Tasman Civil Defence Emergency Management group controller Roger Ball said the flooding was caused by "a bit of a trifecta" with a very low pressure system, king high tides and high winds. "That's driving a lot of water and storm surge into the head of Tasman Bay and parts of Golden Bay."

The weather was forecast to ease overnight, however, another king tide was expected on Thursday night at 11.30pm. It was not expected to be at the same levels – though people should still be prepared.

The West Coast was battered with heavy rain and gales, cutting power to homes, closing schools and shutting down Greymouth's CBD. 

A state of emergency was declared in Buller and Dunedin. 

The capital, Wellington, endured wind gusts close to 140kmh, which toppled trees and power lines, tore down traffic lights, closed highways and halted train and ferry services and dozens of flights. 

Meanwhile, in nearby Christchurch firefighters were battling two fires in searing heat and windy conditions. 

Trees felled by high winds ripped apart power lines and left around 2800 people without power in the South Island, electricity distributor Orion said in a statement.

The country's weather forecaster Metservice said heavy rain and winds would continue as the storm would travel south-east across the South Island until it cleared the country on Friday.

According to MetService, the temperature reached 34.5 degrees just before 1.30pm on Thursday. The city has the highest temperature in the country.

When stormy conditions pelted Wellington, one man decided to take on the elements and swim out to the diving pontoon on Freyberg Beach.

Council spokesman Richard MacLean urged swimmers on Thursday to resist taking advantage of Wellington as "the whitewater-rafting capital".

"The conditions – to say they are challenging would be the understatement of the year," he said.

Surf LifeSaving central regional manager Charlie Cordwell also discouraged the public from swimming at any unpatrolled beaches, particularly when the weather was poor.

Common sense was paramount, as paid beach patrols ended once students returned to school. Volunteer patrols would take place over weekends, he said.

Reuters, Stuff.co.nz