Huge slips have decimated parts of the South Island Main Trunk Rail Line along the Kaikoura coast, following the magnitude 7.5 earthquake overnight.

In some parts, the rail line has been uplifted from its foundations and shunted onto the seabed. The line has shifted sideways by around 100m in some places.

In other areas, the line has completely disappeared under rockfalls along the narrow transport corridor between Conway Flat south of Kaikoura, to Kekerengu, where the line heads inland to Marlborough.

KiwiRail chief executive Peter Reidy says it could be two days before they know the extent of the damage to the local rail network.

"As we send our staff out to inspect, we've got to bring them back if there are aftershocks, and then they'll have to go out again."

There is still no estimate on how long any damage could take to fix, but first impressions suggest it's at least on the same scale as the Christchurch quakes of 2010 and 2011.

As for the cost, Mr Reidy hasn't started thinking about it, saying the focus is on safety.

KiwiRail is working closely with New Zealand Transport Agency to get contractors out to affected sites as soon as possible.

"We've done this before. There are slips in New Zealand that do happen, so we have protocol for how that occurs."

Rail bridges have also been affected, with the rail line buckling on the abutments and approaches.

The pedestrian overbridge at Ava Railway Station in Lower Hutt is closed until it can be confirmed safe for trains to pass under.

"The bridges you don't know until you do your structural inspection, and we're doing those as we speak," says Mr Reidy.

He is confident the Greymouth to Christchurch tourism train will continue operating as normal, but the Picton to Christchurch is shut for now.

With the main trunk line out of action, KiwiRail is hoping keep freight moving by enlisting the help of local coastal shipping operators.

The line linking Blenheim and Picton opened in 1875 after construction was undertaken from Christchurch to northern centres in Canterbury in the 1870s. A line through Kaiapoi, Rangiora and Amberley reached Waipara in 1880.

Newshub.