Dramatic drone footage shows parts of a South Island highway underwater as a massive storm continues to make its way up the country, shutting down internet services and cutting power.

The country's main north-south route, State Highway 1, has also been cut off between Christchurch and Dunedin, while two lightning strikes have cut power to more than 800 homes in New Plymouth this afternoon.

Aerial footage posted by the Timaru District Council has shown the extent of the flooding damage to SH72, with water swamping massive sections of the road just south of the Rangitata Bridge which appears to still be standing despite the torrent of water smashing through.

The video begins at the southern approach to the bridge, on Geraldine-Arundel Rd [SH72], which shows large sections of the road having been chewed up and spat out by the water from the Rangitata River after it burst its banks yesterday.

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Hi Folks, here’s a bit of insight into what we’re dealing with up at Rangitata. The first stage is to get rid of the water, then we’re working to try and create a driveable surface. It’s going to take a while but we’ve got our roading team on it.

Posted by Timaru District Council on Saturday, 7 December 2019

The water is so high sections of fencing bordering the road can't be seen.

The council said the both SH1 and SH72 would remain closed tonight.

Staff would have to wait for the floodwaters to recede so they can assess the damage and begin repair work.

It said it would focus on getting the bridge at Arundel open on a limited basis "as soon as possible".

It was recommending travellers go to Timaru on SH8 rather than Geraldine as there was better access to facilities.

NZTA have been inspecting the flood damage but access had been limited. Due to the flooding and road damage, the bridge is still unable to be assessed.

State Highway 6 on the South Island's West coast was closed from Hokitika down to Makarora, with several slips on the road. Settlements on the highway were cut off from each other because of the damage.

More than 350km of the highway was shut because of the slips and a bridge washout, New Zealand Transport Agency system manager Pete Connors told RNZ.

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"The real problem we've got is between Fox Glacier and Franz Josef - on what we call the Fox Hills ... there's been some pretty substantial slips."

He didn't know how long it would take to repair the roads.

Spark said earlier today "almost all" of its impacted South Island broadband, mobile and landline services have been restored.

Power was also out from Fox Glacier to Paringa, and was likely to remain so for a couple of days.

Powerco have confirmed two lightning strikes in the New Plymouth suburbs of Strandon and Omata cut power to about 840 homes late this afternoon.

The first strike, in Strandon at 4.33pm, initially cut power to 710 homes. However, at 5.15pm only 498 homes were affected.

And 142 homes in the Omata suburb also lost power in the second strike, about 5 minutes later.