New Zealand weather: Potential 'weather bomb' in Southern Ocean, 'severe weather risks' for NZ

Forecasters warn a Southern Ocean storm could become a dreaded 'weather bomb' over Sunday and Monday - bringing "severe weather risks for New Zealand".

The announcement comes on Tuesday from weather forecast company WeatherWatch, which is tracking models.

The low starts in the South Tasman Sea area before tracking south-east (south of NZ). Once over the Southern Ocean it "rapidly develops into a very serious storm (on global levels)", WeatherWatch says.

"A nasty low pressure system is expected to rapidly develop this Sunday and Monday with modelling today suggesting it may reach 'weather bomb' criteria," WeatherWatch says in a release.

A weather bomb - technically known as bombogenesis - is when a depression drops 1 hectopascal (hPa) an hour for 24 hours.

WeatherWatch head weather analyst Philip Duncan has previously described it as "the weather equivalent of having a tyre blow out" where air pressure plummets and "conditions quickly become unstable".

Computer modelling by both GFS and ECMWF shows by noon Monday the Southern Ocean storm could drop to as low as 936hPa.

"This would make this storm deeper than the hurricanes that have recently hit the United States," WeatherWatch states.

"There will also be a huge air pressure gradient between this storm and the high building over south-eastern Australia. In fact, there may be an almost 100hPa range (which is stunning) between the centre of the high and centre of this storm."

How bad is it for New Zealand?

Fortunately, current models show the 'weather bomb' will be out in the Southern Ocean and not tracking directly over New Zealand. Weather bombs in New Zealand tend to cause isolated but severe damage.

But it will still bring a blast of spring wind and cold air.

"The likelihood of severe spring gales and a wintry change are quite possible around parts of New Zealand this Sunday and Monday, with cooler weather lingering in the south until mid next week," WeatherWatch warns.

"Over the next few days we'll continue to monitor this closely but nor'west gales, heavy West Coast rain and some snow and low temperatures in the south are all possible. It's still early to lock in though - so this remains 'One to Watch'."