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A storm the size of Australia is hitting New Zealand with winds of 130km/h around Wellington and heavy snow falling in parts of Westland.
MetService has issued severe weather warnings for the whole country as the storm passes over.
It warned snow could fall to 100-200 metres in south Westland and Fiordland, potentially affecting Te Anau, Manapouri and Monowai as well as higher roads and passes.
Heavy rain is expected about the ranges of Buller and northwest Nelson as well as the central North Island high country, where it could turn to snow above 800 metres.
MetService warned that rivers and streams in these areas as well as Canterbury and Otago could rise rapidly.
Strong or gale-force winds are expected throughout the North Island with severe northwesterly gales predicted about northern and eastern areas, as well as Wellington and the Marlborough Sounds.
Winds of 120km/h are likely in the northern half of the North Island, accompanied by heavy squally showers and thunderstorms in western areas.
There is the possibility of hail in Auckland.
'Line of thunderstorms'
"There is a band of thunderstorms, with the potential to create large wind gusts, edging its way towards New Zealand as we speak," MetService operational manager Steve Ready said.
"The line of thunderstorms is lying just to the west of the South Island and is expected to hit the North Island this afternoon."
Mr Ready said the thunderstorms and wind gusts of up to 130km/h would not be widespread.
"They will pop up here and there.
"The westerlies generated over most of the North Island will be very large and they are going to get stronger as the afternoon goes on."
Mr Ready said the storm would come in waves over the weekend.
"There are several pieces to it. The frontal system moving over the North Island will drop and then surge again tomorrow morning."
Weather contacts:
MetService
Weatherwatch
Getthru
Civil Defence
NZ Transport Agency
AA roadwatch
People are advised to be careful while out and about and to cut down on travel. Those planning to travel on the Desert Rd should be careful of heavy snowfall in the area, and drive to the conditions.
Similar warnings have also been issued to those planning sea trips, while the New Zealand Mountain Safety Council has advised those planning trips in the outdoors to hold off until the weather has eased.
Rough seas
Mr Ready said the seas would be rough on the west coasts of both islands, peaking at their roughest tonight.
"They are expected to drop but will stay high over the course of the weekend."
Mr Ready said swells were expected to be between 4-5m today, and 6-7m in some places tomorrow morning.
He said the weather would not start to clear until the middle of next week.
WeatherWatch.co.nz head weather analyst Philip Duncan said that the weather would not be as severe as a hurricane or cyclone, partly because the low-pressure system would be spread evenly and over a large area rather than tightly wrapped around a small centre.
Auckland Civil Defence warned Aucklanders to stay up-to-date with weather forecasts over the next 24 hours.
Group Controller Harry O'Rourke said Civil Defence and local authorities were monitoring the situation.
He reminded people to check their emergency kits and plans.
Oil platform
Meanwhile the Maari oil platform, which is 80km off the coastline of southern Taranaki, is getting prepared from the impending storm.
OMV New Zealand, who operate the platform, said they have "no current cause for concern regarding the Maari oilfield facilities, and is not anticipating any major impact on production".
In preparation for the storm the company has postponed non-essential maintenance and outside engineering work.
"The Maari facilities include the Wellhead Platform (WHP) and the FPSO Raroa. They have all been designed to cope with a storm of this size, and more," OMZ said in a statement. "The WHP is firmly embedded in the seabed, and rises 30 metres above sea level, well above the projected maximum wave heights in this storm."
- With NZPA
