The rain was torrential in some areas with roads covered by flooding in some places on the West Coast, Waka Kotahi manager Moira Whinham said.
She called on people to stay put if they could, and if not, to "drive to the conditions'.
Waka Kotahi said mud, rock and vegetation closed State Highway 73 at Harris Creek near Turiwhate, west of Jacksons and Otira, before midday on Thursday.
A local road detour was in place, but it would add an hour or so to the journey to or from the West Coast.
People taking the detour could travel via Lake Brunner Road and the Stillwater Road turn-off near Jacksons, from the Canterbury side.
Waka Kotahi expected to provide an update Thursday evening.
Lightning strikes over South Island as thunderstorms sweep in, weather warnings posted
Almost 2000 lightning strikes hit the South Island overnight and on Thursday morning as downpours battered parts of the West Coast.
A local road detour was in place, but it would add an hour or so to the journey to or from the West Coast.
People taking the detour could travel via Lake Brunner Road and the Stillwater Road turn-off near Jacksons, from the Canterbury side.
Waka Kotahi expected to provide an update Thursday evening.
Lightning strikes over South Island as thunderstorms sweep in, weather warnings posted
Almost 2000 lightning strikes hit the South Island overnight and on Thursday morning as downpours battered parts of the West Coast.
West Coast Civil Defence emergency management group controller Te Arohanui Cook told Morning Report surface flooding was likely on the roads, and slips could be brought down later in the day. 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 in Fiordland on Thursday should ease around dawn, but continue in Westland until late afternoon or evening.
