'Big and nasty' storm due to hit NZ

Large areas of New Zealand are preparing for a storm cell "bigger and nastier" than the weather bomb that tore roofs from homes at Wanganui earlier this year.

The air pressure is dropping quickly in a system forming over the Tasman Sea and it is expected to bring gale-force winds, heavy rain and snow to parts of New Zealand tomorrow.

The last weather bomb to hit New Zealand was in March when trees and powerlines were brought down and roofs ripped from houses by gale-force winds.

WeatherWatch.co.nz analyst Philip Duncan said the system currently over the Tasman was unlikely to fit the definition of a weather bomb but was "bigger and nastier than the one we had in March".

People need to be prepared for wild weather even though the storm is most likely to be at its worst before it reaches land, Mr Duncan said.

"It's right on the cusp [of being classified as a weather bomb] but it really makes no difference - it's a very nasty storm," Mr Duncan said.

MetService today issued heavy rainfall warnings for the west of the South Island and central New Zealand, including ranges in Marlborough, Taranaki and Tararua.

"Severe gales" are expected at North Canterbury, eastern Marlborough and the lower North Island, including Wellington, Wairarapa, Hawkes Bay, Taihape and Taranaki.

Further north, gales are expected to turn westerly tomorrow afternoon with the potential for severe gusts, MetService said.

Winds will gradually ease overnight tomorrow.

Heavy snow is expected in parts of Otago, Canterbury and southern Marlborough tomorrow and could possibly lower to about 300-400m, MetService said.

The heaviest falls are likely to be above 500m, especially in south Canterbury where a heavy snow warning for accumulations of 10-15cm is expected above 700m.

"This is a significant event and members of the public are urged to stay up to date with the latest weather information."

Q&A

* What is a weather bomb?

When the air pressure in a weather pattern falls 24 hectopascal in 24 hours

* What happened the last time a weather bomb hit New Zealand?

The last one was in March and caused widespread damage at Wanganui. "Hurricane force" winds brought down trees and power lines and roofs were lifted off homes.

* How often do weather bombs happen?

Only once every one to two years does a weather bomb hit New Zealand