Almost a million lightning strikes have been recorded over the Tasman Sea in the past 24 hours, and the thunderstorms are heading for New Zealand.

MetService has issued an orange heavy rain warning and a severe thunderstorm watch for parts of the West Coast of the South Island, along with heavy rain watches for Nelson Lakes and Fiordland.

“It’s, unfortunately, looking like a very wet day,” MetService meteorologist John Law said.

The National Institute of Water and Atmospheric Research (Niwa) said there had been “well over” 800,000 lightning strikes over the Tasman Sea in the 24 hours until 1pm today.

 “The waves of thunderstorms [are] moving southeast,” Niwa said.

“Thunderstorms will become more widespread and active over much of NZ in the coming days.”

The thunderstorm watch was issued for between 3pm and midnight today. The heavy rain warning, meanwhile, has been in place since 9am and should lapse 6pm on Thursday.

MetService’s Law said 38,000 lightning strikes have already been recorded around New Zealand - most offshore, with 135 on land in the past 24 hours.

The electrical storms should ease throughout the day before picking up in the evening, he said.

Between 200-300mm of rain is forecast for the ranges between Otira and Bruce Bay.

Law said 32mm of rain was recorded to have fallen in an hour near Haast.

“This kind of rainfall could cause surface and flash flooding, hail can damage things like crops and vehicles,” Law said.

“So it is worth keeping that in mind and keeping up to date with the latest forecasts.”

He said a “very active” front was moving up across from the Tasman Sea.

“There’s a lot of warm air at the surface and cooler air aloft, which is making for some pretty intense rainfall,” Law said.