A bridge in the Glenroy Valley, near Murchison, has been washed out.
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Police say that some motorists are putting themselves at risk by ignoring road closure signs.
"The closures are in place due to the dangerous conditions and members of the public are reminded that they must comply with any closure signs," Southern Communications Centre shift commander Inspector John Doherty said.
The Tasman district has been one of the worst affected by the weather with the Civil Defence being activated in response to rivers running at record high level following heavy overnight rain.
Tasman District Council said that the Aorere River was running at eight metres, the highest level since records began in 1977.
The Maitai River was at a seven year flood level, with high tide reached and the river level now trending down.
Flood trapped residents
Fire service spokesman Andrew Norris said the Rai Valley Volunteer Fire Brigade this morning rescued occupants from one vehicle who got trapped in their vehicle by flood water on SH6 at Pelorus Bridge, near Havelock. No one was injured, he said.
Mr Norris said the fire service had been to a "couple of dozen" callouts today as a result of the wild weather.
"There are some people who are trapped in their homes which are surrounded in flood water. It is safer for them to stay where they are," he said.
Regional Civil Defence Controller Jim Frater said "We are continuing to monitor river levels closely. At this stage we are not dealing with too many problems, although there is significant flooding in the Aorere and Anatoki rivers.
"The forecast is for the rain to ease. It will be difficult and damp time for campers."
MetService has issued a heavy-rain warning for Westland, Otago, parts of Canterbury, Buller, Nelson, Marlborough, Wellington and the Kapiti Coast, the Tararua Range, Taranaki, the Central North Island high country and the ranges of the eastern Bay of Plenty.
Click here for the MetService warnings.
"The very heavy rain which has fallen over central and southern New Zealand in the last 24 hours has brought high rivers levels in many places, and river levels will continue to rise this morning.
"There may also be slips, surface flooding and poor visibility in the heavy rain making driving conditions hazardous," MetService said.
Mountain Safety Council chief Executive Officer Darryl Carpenter advised trampers in the South Island to stay indoors, but if they did go out to take particular care around swollen rivers.
"You should never cross a flooded or fast-flowing river or stream. If in doubt, stay out. Trampers and day walkers have lost their lives after being swept away when attempting to cross swollen rivers.
"The best plan is to sit out poor weather conditions, camp an extra night or two and wait until the weather improves. This means taking extra food, a communications device such as mountain radio or personal locator beacon and other equipment appropriate to the terrain and conditions."
Strong-wind warnings
A strong-wind warning has also been issued for Marlborough - excluding the Sounds - Wellington, Wairarapa, the central and lower North Island, Gisborne and Hawkes Bay.
MetService said wind gusts of 130-150km/h were expected in exposed parts of the Wellington and Marlborough, with the strongest likely about 11am. SH at Rimutaka Hill has been closed due to high winds.
Gisborne, Hawkes Bay and the Wairarapa could expect gusts of up to 130km/h and other areas gusts of up to 120km/h.
"Gusts of this strength can cause damage to trees, powerlines and insecure buildings, as well as make driving conditions hazardous especially for motorcyclists and high sided vehicles," MetService said.
The severe gale northwesterlies were expected to ease by midday, MetService said.
"By the end of today the front should lie east of New Zealand, and all severe weather conditions should be over."
Yesterday MetService duty forecaster Oliver Druce said the front would affect most of New Zealand.
But it was also fast-moving and the worst of the bad weather should clear by this evening. "We think this will be a very severe event," he said.
"There is potential for flooding and overflowing rivers from heavy rains and also damage from heavy winds in some places."
Weather Watch chief analyst Philip Duncan yesterday said Auckland, Coromandel and Northland would be on the edge of the stormiest weather.
"However, even winds of 30km/h can make for uncomfortable sleeping conditions in tents and awnings, and winds may be gusting to more than twice that amount - up to 70 or even 80km/h in exposed areas."
Most of the country would enjoy a dry New Year's Eve, he predicted. "Once this weather has cleared by Wednesday morning, we can all go back to having summer weather again."
