A Kiwi weather forecaster is standing by his predictions that the country will feel the effects of a "giant storm" which is set to swing by southern New Zealand.

Philip Duncan of weatherwatch.co.nz yesterday revealed an enormous storm would brew in the Southern Tasman Sea before heading southeast towards Antarctica.

It will bring with it a low air pressure system so deep it's regarded as being associated with the "world's largest storms".

Duncan said it was forecasted to be as low as 940Pa, however this morning that had gone even lower at 929Pa.

However, Duncan has been slammed by the country's weather forecasters who are calling for calm.

MetService meteorologist Andrew James said the prediction was premature, while Niwa's Ben Noll agreed, telling the Herald it was unlikely to be as big as the front which lashed the country last weekend.

But Duncan today remained steadfast in his prediction and agreed that while it was winter and normal to get storms involving heavy rain and snow, this one was looking particularly nasty and people, particularly farmers, had a right to advanced knowledge.

The weather pattern was "quite typical", he said.

"The low that they are talking about is not especially low for the Southern Ocean."

MetService was focusing more on potential heavy rain throughout the country and strong northwest winds for the northern North Island across the weekend.

Next week was "quite far away".

"We're not talking about that at the moment."

However, even Niwa has taken aim at the projection, calling for calm amongst Kiwis.

"Have you heard a report about a storm next week?" it wrote on Twitter. "At this point, there is no need to be alarmed.

"There's a good chance it could be really far south of New Zealand, sparing the country of major impacts."

But Weatherwatch.co.nz responded, tweeting, "You may not be interested in the well being of newborn livestock but we very much care and want to give farmers the best possible advanced warning. Queenstown has snow forecast, so too does Southland and we are in key lambing and calving season. NZ is different to America".

Meteorologist Ben Noll, of Niwa, told the Herald that as far as he was aware, the major lambing season kicked off in September.

He added that while there would be snow and rain with Monday's system, it would hit the usual high country areas and was not likely to be any bigger than what shook the country last weekend.

You may not be interested in the well being of newborn livestock but we very much care and want to give farmers the best possible advanced warning. Queenstown has snow forecast, so too does Southland and we are in key lambing and calving season. NZ is different to America.

— WeatherWatch.co.nz (@WeatherWatchNZ) August 6, 2019

"Comparing the two I think that what we saw last week would have been a larger impact to the country on a whole, so that's where it stands right now, at least that's our opinion."

The storm as forecasted brewing in the southern Tasman Ocean at noon on Sunday. Image / weatherwatch.co.nz

Noll said it was common for weather systems to occur in the Southern Ocean, south of New Zealand, all throughout the winter season.

"Really what we're seeing for next week is maybe a slightly beefier version of [last weekend's], passing well to the south of the country ... we're talking 60 degrees south latitude [1000km] ... in that Monday, Tuesday time frame of that week."

He agreed it was "impressive" but not too disturbing at this stage.

"So, the massive storm narrative doesn't really seem to be on the cards, at least in our view at this point in the game. This weather system may flick some chilly air northward in that early to mid week time frame but nothing that is completely outlandish for the time of year.

"It is winter after all."

But Duncan told the Herald that he was aware of lambing and calving having already begun around some parts of the country.

"We've got this enormous area of low pressure forming, and enormous is not a word I use every day, but this is particularly big. The positive is that it will be tracking south of the country and not over top of us but because it's so enormous we will be under this low pressure system for a time.

"So this weekend looks kind of mild with windy westerlies and rain developing on Sunday for most places in the west, but then next week when the low deepens and moves away from us it dredges up this big cold southerly which could be quite a serious concern for farmers in the South Island in the middle of lambing and calving."

The storm as forecasted brewing in the southern Tasman Ocean at noon on Sunday. Image / weatherwatch.co.nz

Duncan claimed Niwa didn't know what it was talking about.

"It's spoken like an American who knows nothing about farming. They're climate scientists not weather forecasters ... I would trust MetService 10-fold over Niwa any day for an update on severe weather and I guarantee that MetService puts out severe weather warnings for this lot.

"I would be so surprised if they don't put anything out about it."

- additional reporting: Otago Daily Times