NZ weather: 'Significant wintry blast' to hit Monday after possible offshore 'weather bomb' sends severe, cold gales our way

New Zealanders are being warned to prepare for a "significant wintry blast" on Monday that will follow a possible offshore "weather bomb" this coming weekend.

Weatherwatch says southern parts of the country should expect a cool chill on Monday as the "biggest pool of Antarctic air" to hit New Zealand this year moves north.

The weather forecaster says farmers with newborn lambs and livestock in Southland, Otago and the West Coast should be on alert.

"Southland Farmers - prepare now," Weatherwatch says in a tweet. 

It's expecting snow flurries in Gore, Lumsden, Queenstown, Arrowtown, Wanaka and potentially further east in Dunedin. Sleet is also possible in Invercargill with "below zero wind chill at times". 

"Daytime highs will range from just 2C to 7C across the lower South Island at the warmest point of Monday, based on today’s data. Wind chill will likely be in the negatives for a large portion of Monday in exposed areas of Southland. This could be deadly for newborn lambs."

Weatherwatch says it could be one of the "lower to sea level snow events" of the year, despite it being September. 

"Late snow storms in September and October do happen from time to time – Spring is all about winter slowly fading away and hints of summer slowly coming in."

But Monday's Antarctic blast won't be the only chilly weather Kiwis will have to possibly contend with in the coming days.

"A rapidly deepening storm will track past NZ this weekend and grow into a serious storm over the Southern Ocean across Sunday and Monday, sending severe gales and cold winds back into NZ," Weatherwatch says.

The South Island and central parts of the country around the Cook Strait will be the most impacted by winds and cool air, but the north could also see gales. 

Weatherwatch says the low could reach criteria to be classed as a "weather bomb", which it says is when air pressure falls more than 24hPa in 24 hours.

Metservice says in its severe weather outlook that, on Saturday, a front is approaching the country from the Tasman Sea "preceded by strong to gale northerlies". 

"This front is expected to move east over New Zealand during late Saturday and Sunday, bringing heavy rain to some western areas and snow about the South Island ranges," Metservice says. 

"A showery westerly flow follows this front, turning to cold southwesterlies on Monday, as a trough moves north over the South Island bringing snow to low levels in the south."

On Saturday and Sunday there is "moderate confidence" that rainfall accumulations reach warning amounts around the West Coast, from Fiordland to Buller, Otago and the Canterbury headwaters. 

"Also on Sunday, there is a low confidence of heavy rain about Nelson and the Marlborough Sounds, including the Richmond Range, also Taranaki, the King Country, the Central Plateau and eastern Bay of Plenty Ranges."

In terms of wind, there's a possibility of "severe northwesterly gales" about the Wairarapa on Friday. From late Saturday to Sunday, there's also a chance of severe north to northwest gales over the North Island as well as northern parts of the South Island.

"Although this weekend looks like a real doozy, don't forget about today," Metservice said on Thursday morning.

It currently has a heavy rain watch in place for north Taranaki, Waikato south of Hamilton, eastern ranges of the Bay of Plenty and Fiordland, north of the Breaksea Sound. A heavy rain warning is in place for Waitomo, Taumarunui, far west of Taupo, and the Tongariro National Park.