The houses were spared but in the dark, residents evacuated, with help from neighbours.
At Kaiata, a car was stuck in floodwaters but the driver got themselves out.
The downpour also caused surface flooding on Main South Rd by Panthers Rd, and closed the Rutherglen underpass.
At Barrytown, south of Punakaiki, the highway remained closed yesterday morning, but was expected to reopen yesterday afternoon. It later reopened under stop-go management.
Metservice meteorologist Alwyn Bakker said about 1700 lightning strikes were counted in the Greymouth area between 10pm and 7am.
"[There was] both cloud-to-cloud and cloud-to-ground lightning. There were about 3500 strikes detected by the lightning network over all of (New Zealand) in that time, so you got about half of them."
The NZ Transport Agency said SH6 was closed from Barrytown to Punakaiki due to debris across the road.
"Crews are working to clear the debris and redirect the creek back through the culvert it usually runs through, no other damage concerns other than a couple of small slips and the usual surface flooding."
The thunderstorm dumped 118mm of rain in Moana in the 24 hours to 7am, though most of that was overnight.
Greymouth got 87mm, Hokitika 84mm and Kumara 93mm.
Most South Westland towns escaped the heavy falls, receiving just 30mm.
A neighbour of the homes evacuated at Barrytown, who declined to be named, said the slip was at the next creek past Little Granite Creek.
"There has probably been a slip in the hills, which blocked it, and it all come down in a big whoosh — logs and boulders."
He said residents heard the rumble coming.
He felt it would take a lot of work to get the creek back in its original course, and said it was quite wet around one of the houses on the other side.
One of the evacuated families heard "two loud rumbling noises".
The houses were not damaged but the occupants evacuated as they could not tell in the dark where the slip was coming from, he said.
Richard Reynolds said the creek had completely changed course.
"It is now on both sides … gone around houses."
Meanwhile, Runanga and Cobden crews were both turned out at 5am when the fire alarm at Runanga School was activated.
Crews inspected the building and found nothing at fault, and attributed the alarm to a huge clap of thunder and lightning strikes over the town at the time.
The downpour continued and Runanga crews carried out a sweep of the town to find serious surface flooding on the corner of Hall Jones and Carroll streets.
Raleigh Creek was also very high, and water was pouring out of the stormwater manholes.
Deputy fire chief Aaron Sheehan said the weather had caused damage to roads in the area, lifting tarseal.
Greymouth and Brunner fire crews responded at 5.30am to a car swamped by floodwaters on the Kaiata Straight.
The occupant was able to self extricate and no action was required by brigades.
At 7am, residents on Main South Rd dialled 111 amid concerns of water entering houses when a culvert blew out and water ran over some properties.
At Paroa School, the car park was again under water, a repeat of flooding during the Labour Weekend storm.
- Meg Fulford and Laura Mills, Greymouth Star
