The MetService has recorded over 1970 lightning strikes over the South Island since late last night.
It issued a severe thunderstorm watch for Westland and Fiordland north of George Sound up to 9am.
Orange heavy rain warnings cover Westland and the headwaters of Canterbury and Otago lakes and rivers until this evening, and Fiordland north of Doubtful Sound to 7am.
The Westland region was forecast to have up to 240mm of rain in the ranges by this evening, with 100mm-140mm expected near the coast.
Some thunderstorms may be severe with localised downpours of 25-45mm an hour and hail up to 20mm in diameter, MetService said.
There may be one or two small tornadoes near the coast, which cause some localised damage to buildings and vegetation, and strong wind gusts. The forecaster warned there could be surface flooding, flash flooding and slips.
By early morning there had been no river level warnings, she said.
Thunderstorms were bringing localised bursts of heavy rain rather than continuous downpours in one spot.
"In Hokitika we're going from light rain to short heavy showers but we are aware that they had heavier falls in South Westland from Hari Hari down to Haast."
Thunderstorms were mostly off the coast but had the potential to come back on to land.
With the weather due to clear before the weekend, Cook said, the Wildfoods Festival at Hokitika was still on track.
MetService said thunderstorms should ease in Fiordland around dawn, but continue in Westland until late this afternoon or evening, and the severe thunderstorm watch for Westland may be extended.
Almost a million lightning strikes over Tasman
Almost a million lightning strikes have been recorded over the Tasman Sea in the past 24 hours.
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.”Earlier, a 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.
The MetService said 38,000 lightning strikes have already been recorded around New Zealand - most offshore, with 135 on land in the past 24 hours.
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.
