MetService's rainfall forecast for Thursday afternoon shows heavy rain in the South Island - many areas between Dunedin and Christchurch could get close to their normal July rainfall, with some areas possibly getting up to double that.

A storm bringing snow and torrential rain to the country at the end of the week could see some areas getting twice their monthly rainfall in one day.

The blast was forecast to bring downpours and gales to the country and snow down to 400m in some parts of the South Island.

Niwa principal scientist - forecasting Chris Brandolino said many areas between Dunedin and Christchurch could get close to their normal July rainfall on Friday and through to early Saturday. Some areas could get up to double that.

MYTCHALL BRANSGROVE/STUFF Rain clouds gather in South Canterbury.

"Most locations from Dunedin to Banks Peninsula will see circa 50mm from this rain event," Brandolino said.

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"I have high confidence that the greatest rainfall events will be near or above 100mm by 6am Saturday."

HAMISH MCNEILLY/STUFF Snow closed some Dunedin roads last week - and more is picked for this week.

He would not rule out the possibility of some areas getting as much as 150mm.

While that was heavy rain, the normal July rainfall in much of the area between Christchurch and Dunedin was considerably lower than in many other areas of the country.

The heavy rain was the result of a low with a tropical connection coming out of the Tasman Sea.

"The low is helping to draw moisture from the subtropics and the tropics," Brandolino said. That moisture would find its way into the South Island.

"The moisture content of this plume of air coming from the sub tropics and tropics will provide the region with atypically high levels of moisture. When you get atypically high levels of moisture in an area where it doesn't belong you are going to get atypical results."

Earlier MetService meteorologist Tom Adams said rain would exceed 100 millimetres in Christchurch on Friday, which was nearly a sixth of the city's average annual rainfall. The Port Hills could get a normal month's rain in a day, he said.

By Friday, heavy snow could fall above 400 metres in the South Island and the heavy rain could reach southern Marlborough. Strong southerlies could mean snow about the hilltops in Dunedin and snow flurries in Queenstown.

The rain or showers would continue in most other places.

Severe northerly gales were possible in Gisborne and northern Hawke's Bay during late Thursday and early Friday, with a low risk of severe southerly gales in exposed places of Wellington and southern Wairarapa on Saturday.

There was a low risk of severe southerly gales about coastal Canterbury and the Kaikoura Coast during Friday and early Saturday.

CALM BEFORE THE STORM

For Wednesday, MetService predicted showers and settled spells for many parts of the country.

In Wellington a few showers were forecast to develop around midday, with northerlies and a high of 12C.

Auckland could be showery from late morning on Wednesday, with northerlies developing and a high of 16C.

Christchurch was expected to start the day with fine weather and morning frosts, but high cloud is likely to develop in the afternoon, with northerlies and a 14C high.

Dunedin was expected to have a similar Wednesday to Christchurch.