Is it a UFO, is it a fire, or is it lightning? The question of mysterious flashing lights reported all over Northland has been put down to a simple electrical storm.

Inquisitive Northlanders took to social media last night after reportedly seeing a "very big flashing light in the sky" or "intermittent light flashes".

While some put it down to a UFO or a fire, Metservice Meteorologist Amy Rossiter confirmed there is a trough lying to the east of Northland which has had quite a few lightning strikes.

"In the past 24 hours there has been about 32 strikes. There were a few yesterday evening on land, but overnight we haven't seen anything on land, most of it has been offshore to the east," she said.

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Rossiter said this lightning storm extends all the way from offshore Bay of Plenty all the way up to Northland.

She said reports of the flashes being described as red or orange in colour could be attributed to the way they are reacting to various clouds or things in the atmosphere.

It's also almost time to bring out the winter woollies, with lows reaching -5C and rain expected for most of the country.

Rossiter said a ridge over the South Island has caused temperatures to drop, with a few places falling into the negatives and getting frosts.

"In the Central Otago, Canterbury High Country areas, we had temperatures as low as -4.8C, so it's pretty cold down there.

"Alexandra was -2.5C overnight, and Tekapo was -3.2C, while Queenstown, Wanaka and Invercargill all got down to 0C overnight," she said.

A heavy-rain warning is in place for eastern Marlborough, especially about the Kaikoura Coast and ranges, until 1am tomorrow.

Heavy-rain watches are also in place for Hawkes Bay, mainly about the coast and Wairoa district, eastern and southern Wairarapa, and the eastern hills of Wellington, especially the Orongorongo Ranges.

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Rossiter said in the past 24 hours, the highest rainfall rates were recorded in Wainuiomata with 99.6mm, while Wellington city has had about 30mm, with peak rainfall rates of 20mm/h.

This morning there is a moderate risk of thunderstorms about eastern and northern Marlborough, Wellington, southern Wairarapa and also about coastal Gisborne and over Mahia Peninsula.

A lower risk extends to north Canterbury, remaining areas of Wairarapa and coastal Hawkes Bay and also about coastal parts of Whanganui and south Taranaki.

Rossiter said there are also a few troughs moving over the North Island, so Northland and Auckland will get a few showers later today, but most of the South Island will remain fine but cool.

The rain is expected to ease during Monday as a ridge over the South Island extends onto central New Zealand.

However, rainfall accumulations may reach warning criteria in coastal Hawkes Bay and the Wairoa District during Monday.

On Tuesday a front is expected to bring more rain to the south of the South Island from the west, and move up the country on Wednesday and Thursday.

This front is forecast to bring northerly rain to the west of the South Island, with a chance of rainfall accumulations reaching warning criteria in Fiordland and central and southern Westland during Tuesday, Wednesday and Thursday, also affecting the southern Alps during Wednesday and Thursday.

Today's forecast

Whangarei:

Cloudy periods. Chance shower. Southeasterlies, turning southwest evening. High 20C / Low 10C.

Auckland: Cloudy periods and a few showers. Southeasterlies, turning southwest this afternoon. High 18C / Low 11C.

Tauranga: Some morning cloud, then fine. Southerlies. High 20C / Low 10C.

Hamilton: Some morning cloud, then fine. Southerlies, tending southwest afternoon. High 19C / Low 5C.

New Plymouth: Morning cloud, then fine. Southeasterlies. High 17C / Low 10C.

Napier: Cloudy periods, chance shower. Rain developing this evening with possible thunderstorms. Southwest breezes. High 18C / Low 11C.

Wellington: Rain, with heavy falls and possible thunderstorms. Brisk cold southerlies. High 15C / Low 12C.

Nelson: Cloudy periods, chance shower. Southerlies. High 17C / Low 8C.

Christchurch: Cloudy periods, a few showers. Southwesterlies. High 14C / Low 7C.

Dunedin: Fine. Northeast winds. High 16C / Low 10C.