NZ to be hit by worst storm system in 50 years

Heavy rain and high winds forecast for a big swath of the country were set to cause big disruption on the last day before the Easter break, with the NZ Transport Agency (NZTA) warning it may have to close the Auckland harbour bridge if wind speeds get high on Thursday afternoon.

Air travellers looked certain to be affected too with Air New Zealand advising customers to anticipate delays and cancellations across its network.

Both Bay of Plenty and Coromandel have declared states of emergency, ahead of Cook's arrival.

A Thames District Council spokesperson said civil defence were currently making a decision about whether or not people in low lying areas of the Coromandel would be evacuated.

Rain overnight in most of the country ??? including heavy rain which may have caused flooding in Auckland and Northland ??? was just a taste of what's to come later on Thursday, with Bay of Plenty and Coromandel set to be hit the hardest by downpours and severe gales.

Just before 5am, the ex-tropical cyclone was about 350km northeast of the North Island and moving south, MetService severe weather forecaster Gerard Barrow said.

It was expected to make landfall around western Bay of Plenty or Coromandel late afternoon or early evening, bringing "quite a significant amount of rain and strong winds" to northern parts of the country.

"Already ahead of the cyclone there is quite a broad rain band which is also spreading over northern New Zealand, so that's going to help keep the rain there today," Barrow said.

The cyclone would continue south, to lie over the lower North Island about midnight on Thursday.

"It's going to bring quite a lot of significant weather to many parts of the country, especially rain and severe gales, especially over the North Island," Barrow said.

"I don't think many people are going to spared from the weather associated with this low, but the places that are going to be impacted the most will be Bay of Plenty and the Coromandel Peninsula."

Flooding was likely in those places, including the flood-stricken Bay of Plenty town of Edgecumbe, while severe gales could cause structural damage.

Barrow himself had never forecast a storm as big as Cook, which was expected to be similar to Cyclone Giselle, which sank the Wahine in Wellington's harbour in 1968.

"A lot of people have been comparing it with [Giselle]. This is very definitely a significant storm.

"I've worked here for 20 years and it's most probably the first time that I've actually had to forecast for such a system which has the potential to produce such a great impact on the country."

With winds forecast to hit severe gale strength in Auckland, NZTA was warning motorists to plan ahead for the possible closure of the city's harbour bridge ??? an unprecedented step.

"Closing the bridge is not a decision we will take lightly because it will have a significant impact on traffic flows throughout the city, however our first priority is safety and that's what will determine any closure," Auckland Highway Manager Brett Gliddon said.

"If the bridge is closed to traffic this will have a flow-on effect across the rest of the motorway and local road network, so motorists should expect long delays, and avoid travelling if they don't need to."

Thames-Coromandel Civil Defence controller Garry Towler told RNZ people in low lying areas were being told to move to higher ground.

"We are gathering our teams together to go around the [Coromandel] peninsula and evacuate everyone we can from low-lying areas and get them into our Civil Defence centres - which we will open later this morning in preparation for a noon arrival," he said.

These low-lying areas are mostly coastal where there are holiday homes and baches. He doesn't know numbers, but says "we're not talking hundreds and hundreds of people".

"We're still encouraging everyone in low-lying areas to get to higher ground. When this storm arrives it's going to bring a 3 - 5m storm surge."

While the storm was still a way off New Zealand, heavy rain had already begun in some regions.

Northland saw the most rain. In the 12 hours to 6am Thursday, one station near to Kerikeri saw 137mm of rain. Kerikeri itself saw up to 30mm of rain an hour, a serious downpour.

Stations in Auckland received up to 73mm of rain in the 12 hours to 6am. One station received 27mm of rain in one hour early on Thursday morning.

The Coromandel received 58mm of rain over the same period, with peaks of 14mm an hour.

Strong winds had brought down a number of trees across the Bay of Plenty and Coromandel overnight Thursday.

The Ministry of Education has advised all schools and early childhood centres in Coromandel and dozens in Bay of Plenty to close on Thursday.

Fairfax Media, New Zealand