Weather: Parts of Northland, Auckland told to brace for thunderstorms, flash flooding as New Zealand settles in for week of rain

Aucklanders face a gloomy start to their alert level 3 COVID-19 lockdown, with the region set for cloud, rain and even a thunderstorm on Monday afternoon.

Rain is forecast for large swathes of New Zealand this week, with widespread, persistent showers and cool, windy conditions marking our first week of autumn.

MetService has issued a Severe Thunderstorm Watch for western parts of Northland and north-west Auckland from 2pm. The forecaster warns these downpours have the potential to cause surface and flash flooding in isolated areas.

Weather Watch says heavy showers will also likely develop on Monday in the North Island's inland areas before petering out by evening.

North-west Nelson is another region in the firing line - the city is set for five hours of heavy rain, possibly creating dangerous road conditions. The South Island will see showers in the West Coast, Buller and Tasman too.

"In the east conditions are drier - expect plenty of cloud though, especially this morning. About Southland and Otago isolated showers develop this afternoon especially inland, some may be heavy with a risk of thunder then clearing tonight," Weather Watch says.

The soggy weather comes as a high pressure system holding off fronts coming towards the country from the west "gives up the fight", with a low bringing rain able to make landfall as a result.

NIWA Weather warns that while the week will start warm - thanks to air flowing from the tropical Coral Sea - that will give way to much colder, windier weather this coming weekend as sub-antarctic winds kick in.

The forecast means western parts of the South Island and northern parts of the North Island will get plenty of rain this week, with those elsewhere enjoying mainly dry conditions.

Tuesday and Wednesday will bring showers for most parts of the country except the likes of eastern-lying Gisborne, Wairarapa, Wellington and Christchurch, MetService says.

Rain will be there on Thursday and Friday too, though most persistent in western areas, with Gisborne and Hawke's Bay missing the worst of it.