Surface flooding and minor rockfall has forced the closure of State Highway 1 north and south of Kaikōura until further notice.

NCTIR journey manager Tresca Forrester said the overnight downpour had caused debris to fall onto the highway.

Crews were out clearing up the roads of loose rock.

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"We've got our sweepers out clearing the road of any loose rock or debris," she said.

Forrester couldn't say how many slips had come down, but crews were keeping a watchful eye on a site of interest north of Kaikōura

"We've got a site in the north that we're just watching which is one of our known sites at Waipapa Bay, so it's a slip that's contained by a big retaining wall where the single lane section is up there.

"We're just keeping a close eye on it at the moment."

Forrester said the rain had also blocked drains along the route, causing some surface flooding.

Geotech engineers were inspecting the road and slip conditions to determine when the road would be safe to reopen.

Forrester said it was possible the road could still be closed until Saturday.

"We could be in the same situation tomorrow morning," she said.

Severe weather warnings were in place across northern and central New Zealand as well as the top of the south, including Marlborough and the Kaikōura coast.

Trains had been forced to stopped running along the Main North Line between Picton and Christchurch, with the line to be reassessed at the weekend.

The inland route to Kaikōura via Route 70 was open this morning.

NZTA would release another update on the road at 7am on Saturday.

Police senior sergeant John Price said motorists should "take it easy on the roads".

"Conditions like this can make driving more difficult. For that reason, we want motorists to plan ahead and avoid driving unless absolutely necessary.

"Allow extra time for your journey and remember - the speed limit is not a target, it's the maximum legal speed that you can travel at on a road in ideal conditions - and they often don't exist during weather events such as this."