Roads are cut off while evacuations are continuing across the top of a deluged South Island, while Auckland braces for an afternoon of potential thunderstorms.

A tornado blasted through Auckland this morning, with a building collapsing in the high winds, while torrential rain continues to buffet the city.

Many households are without power and drivers on the eve of school holidays have been warned of treacherous conditions on the roads.

It comes as Nelson and Marlborough declare states of emergency, with dozens of homes being forced to evacuate due to rising floodwaters.

Northland and Auckland are under a severe thunderstorm watch this morning, which is expected to lift early this afternoon, MetService said.

NZ Transport Agency Waka Kotahi (NZTA) said speed limits had been reduced on the Auckland Harbour Bridge as 90km/h winds whip through the city this morning. Drivers should also be prepared for possible lane closures between 6am and 10am.

Wind gusts could trigger an amber alert, NZTA said.

Intense rainfall in northern regions could cause surface and/or flash flooding, especially in low-lying areas such as streams, rivers or narrow valleys, and may also lead to slips.

Local state of emergency issued 

A local state of emergency for part of Marlborough was issued by Mayor Nadine Taylor last night as flooding fears rose.

The Wairau River stopbank at Spring Creek is currently in a compromised state, so Marlborough Emergency Management advised residents to plan to evacuate from 6am.

The streets affected by this precautionary evacuation in Spring Creek are Dodson, Hathaway and March Sts and part of Ferry Rd.

A Civil Defence Centre to provide community support and information will open for evacuated residents.

Stadium 2000 in Blenheim will open from 9am and Tuamātene Marae at 2518 State Highway 1, Grovetown, will also be open at 7am.

A local state of emergency for part of Marlborough has been issued this evening as flooding fears intensify over a compromised stopbank. Photo / Marlborough District Council 

Residents are encouraged to stay with friends and family as their first port of call. 

Marlborough Emergency Management controller Richard MacNamara said, “All of Marlborough’s emergency and lifelines agencies are up to speed, and our response teams are ready to go.” 

Watches and warnings across NZ 

Most of the country has been put under a severe weather watch or warning this morning. 

Auckland and Northland are both under severe thunderstorm watches. 

These regions could see rainfall rates of up to 40 mm/h. 

Bay of Plenty, Rotorua and the Tasman District northwest of Motueka all face a heavy orange rain warning today. 

“The Nelson and Tasman Region has had significant rain recently, and we’re expecting a lot more to fall by Friday evening,” MetService meteorologist Michael Pawley said. 

Before the end of today, northern parts of the South Island will likely see more than a month’s worth of rain. 

Dunedin and North Otago are under a heavy rain watch, with forecasted periods of heavy rain. 

In addition, snow is expected to affect parts of inland Otago and South Canterbury through Friday night and Saturday morning.