Cyclone Oma to fuel 'unpredictable' severe weather in New Zealand

Cyclone Oma might look "less likely" to strike New Zealand but will still fuel bad weather.

Forecasters at Weather Watch warn an "unpredictable" low is going to form on Sunday, creating pockets of "severe weather" - including gales, heavy rain, and big seas.

The Severe Category 3 tropical storm is currently moving from the Coral Sea. Updated computer models say there's only a 25 percent chance of it brushing the northern regions.

But as it heads down, it's pulling down a lot of tropical energy closer to New Zealand. At the same time an area of high pressure east of New Zealand will pull down more sub-tropical nor'easters.

"While Oma isn't likely to directly hit NZ, it may help fuel energy into a new system that is coming out of the Southern Ocean," Weather Watch says.

"We expect a large area of low pressure to form over New Zealand this weekend and it may deepen rapidly to the east, creating heavy rain and gales in some areas."

This means it's likely Aotearoa will be battered by a wild mix of weather.

"If a deep low does form just east of central NZ it could create gale easterlies on the West Coast, heavy rain on the east coast and lower North Island and may be some heavy rain in northern NZ where it has more tropical energy," Weather Watch warns.

NIWA forecaster Ben Noll told Magic Talk's Peter Williams it was possible the system could bring flooding and destruction similar to that which Cyclone Gita delivered last year.

"The reason that could occur is because the soil right now is very dry, and that means it is kinda like concrete, so when the rain falls on that soil, that may just run off and not get absorbed by the ground," Mr Noll said on Tuesday.

Temperatures are also likely to fall, with some parts of Southland, Otago and Canterbury falling 20degC from recent highs.

"Overnight lows in the upper North Island look set to drop, from the low 20s for a time this week to around 12 and 13degC by next Monday in Auckland," Weather Watch NZ says.

"Sunday and Monday daytime highs around a large portion of Southland, Otago and Canterbury will be just 11 to 14 degrees at the hottest part of the day. In recent weeks these places have highs in the low 30s at times."