Weather: Heavy rain across New Zealand, thunderstorms, hail possible

Keep a raincoat handy - that's the advice from forecasters as a cold front moves over the North Island on Wednesday bringing cold, unsettled weather and a chance of thunderstorms.

Weatherwatch on Tuesday warned "two surges of severe gales" will hit the country this week along with heavy rain and snowfall. 

The first arrived on Wednesday morning, with a large rain band pushing over the North Island from the west, while a Strong Wind Watch is in place from 11am to 7pm for Auckland, the Great Barrier Island, Coromandel Peninsula and northern Waikato.

"Rain develops in the morning as northwesterlies change strong southwest with gales likely especially out west, there could be a rumble of thunder," Weatherwatch said on Wednesday morning.

"A second burst of heavy showers moves through around midday Auckland southwards with a chance of thunderstorms and hail then easing evening and winds start to ease too. Becoming dry in the evening for the Bay Of Plenty and some other eastern spots."

On the west of the central North Island, morning rain will ease before "heavy showers in the afternoon with thunderstorms and hail possible". 

"Northwesterlies change gusty southwest in the morning, gales about the coast in the afternoon then easing later in the day. Snow lowers to 700 or perhaps 600m."

It will be a mostly sunny morning in the east with just a few showers developing about Wairarapa before they spread northwards in the afternoon. 

"Wairarapa may see heavy showers with a chance of thunderstorms and hail for a time. Snow lowers to 600m before clearing."

In Wellington, Weatherwatch is forecasting showers and strong northwesterlies that will change into a southerly mid-morning. Rain may become heavy around midday with a risk of hail.

Nelson will see showers clear in the afternoon, but Marlborough may have to contend with thunderstorms and hail into the evening.

Further south, in Canterbury, morning showers may turn heavy with hail before easing in the afternoon. Across on the West Coast, heavy rain in the morning will eventually be broken by sunny spells. 

"Later in the evening rain with heavy falls pushes into Fiordland then spreading northwards overnight. Snow to 300m in the south, 600m in the north, lifting later in the day. Strong southwesterlies gusting to gale about the coast then easing at night."

Metservice has a Heavy Snow Watch in play for Fiordland south of Te Anau from 3am to 6pm on Wednesday. 

"Snow is expected down to 200 metres during this time, with heavy falls above 400 metres. Snow amounts may approach warning criteria above 400 metres," it says.

Southland and Otago will see heavy rain and hail through into the afternoon before it dries out in the evening with strong winds reaching gale levels near the coast.