Areas were also thrust into darkness as high winds brought down power lines.
The remote Haast network — which is not connected to the national grid — took another hit with power not expected to be back on until after the long weekend.
NZ Energy said the "atrocious weather conditions" had broken a pole south of Hannahs Clearing, which is about 20km south of Haast. Repairs were being delayed by continued bad weather.
"This outage may be several days."
A slip at the Gates of Haast closed State Highway6 in the morning and the road was also under water at Pleasant Flat. The road between Haast and Makarora remained closed late last night.
An emergency operations centre was opened at the Westland District Council headquarters in Hokitika as the storm intensified.
It issued an urgent notice late in the afternoon warning that Greymouth’s Omoto reservoir had dropped to a critically low 10%.
Domestic water supply areas affected were Greymouth, Cobden, Blaketown, Boddytown, Karoro, Paroa, Kaiata, Dobson, Taylorville, Stillwater, Runanga, Dunollie and Rapahoe.
"This is due to a storm-related issue at the greater Greymouth water treatment plant," the council said in a statement.
A large team of workers were on site at the council’s Coal Creek treatment plant and had been working for "several hours".
Earlier in the morning, Grey district from Gladstone north had no power for more than two hours.
While supply was restored in Greymouth and as far north as Kaiata by 9.40am, lines company Westpower warned residents to expect intermittent outages yesterday.
This was due to the "switching" process to gradually and safely restore electricity through the local network after a Transpower issue in the Nelson region, Westpower network manager Mark Blandford said.
National grid operator Transpower had identified the issue affecting the northern West Coast and Nelson-Marlborough by midmorning. This was on the Transpower feed from Christchurch via Culverden to Nelson, from which the northern West Coast receives its electricity via Murchison and Inangahua.
"As far as we know, we lost the three circuits, which is what created the loss of supply," a Transpower spokesperson said at 10.43am.
The storm remained an issue as Transpower tried to resupply local networks through yesterday.
All three alpine passes to the region remained closed due to high winds, washouts or slips.
A Coolpak truck-and-trailer unit flipped about 9am just west of Springfield, closing the road.
State Highway 7 between Springs Junction and the Hanmer Springs turnoff is also closed due to the washout of the road near Engineers Camp.
In the Greymouth CBD, wind gusts snapped a level crossing barrier arm at the Mackay St rail crossing.
The Grey District Council advised yesterday sandbags were being made available to the public from the Preston Rd depot "as a precaution".
As the storm moved northeast, the council warned Grey Valley residents to stay indoors as high winds felled trees. Slaty Creek and Shellback Rd at Moonlight were open, but affected by fallen trees.
Trees were also down in Aratika Dr and Beechwater Dr towards Lake Brunner.
The Iveagh Bay rail underpass had flooded.
Members of the public cleared several trees off the Taylorville-Blackball road about 11am.
