The section of line running through the Buller Gorge has been particularly hard hit, with the difficult terrain in the area hampering access by track workers to assess the situation and begin the repair job.
Several bridges had been damaged, one significantly, and slips and washouts had damaged track at a number of locations, a spokeswoman said.
It is anticipated the line between Christchurch and Greymouth will be back in operation within three to four days, allowing the resumption of the Tranz Alpine and freight services across the Southern Alps.
Coach services are in place for passengers with bookings on the Tranz Alpine.
However, the section through the Buller Gorge will take longer for repairs to be completed, and coal services from the Westport area may not resume for at least a week.
KiwiRail is in close contact with Solid Energy over the situation.
Meanwhile, the New Zealand Transport Agency (NZTA) says Wanganui bridge will be closed at least until midday next Tuesday (JAN 8).
"Next Tuesday's our target providing we get the Wanganui River diverted and that there is no more bad weather,'' said NZTA senior asset manager for the West Coast Mark Pinner.
"We are aware of how important this link is to the coast and others regions and we're confident we'll get the highway re-opened as planned if there are no setbacks.''
Flood waters from the river swept away a 40m-long of SH6 at the northern approach to the bridge on Wednesday.
"Water levels have dropped about a metre overnight and that's helping us divert the river back to its original course so that we can start out preparations to rebuild the highway lost in the flood,'' Mr Pinner said.
The river is still too high for structural engineers to make a detailed inspection of the bridge, but Mr Pinner was confident it was safe.
"The piles supporting the bridge have been driven deep into the river bed and from what we can see already, they appear to have withstood pressure from the huge volume of flood water. However, we will still be taking a very close look at the bridge when levels drop further so that we are satisfied it is safe to use.''
Contractors are using heavy earth moving equipment to lift large boulders along the edge of the Wanganui River to redirect it.
Mr Pinner said the lost section of the highway would be rebuilt with rocks and gravel.
"The reconstruction will be temporary one and further upgrades will be needed later,'' Mr Pinner said.
"We recognise that this closure is affecting the lives and livelihoods of the tourists, transport operators and farmers who rely on this lifeline.
"Getting SH6 reopened is our number one national priority and we thank people for their patience and understanding while we finalise our plan of attack.''
It's not just travel plans that have been disrupted by the weather.
Around 50 dairy farmers south of Hokitika have been forced to dump milk because the road north was cut off.
Westland Milk Products hoped to catch up with its South Westland milk collection within 24 hours.
Some milk had to be spilled into farm effluent disposal systems but the company was hopeful the majority would be collected in the next few days using a fleet of tankers travelling the long way around from Hokitika (1000km via the East Coast and Haast), with the help of extra tankers supplied by a contract collection company.
General manager operations Bernard May said Westland was very grateful for the "very supportive response we have received from fellow dairy companies which have agreed to process our milk so we don't have to truck it all the way back to Hokitika''.
"This means we can get a faster turnaround so we are hoping that within the next 24 hours or so we will have caught up on the backlog and prevent more milk having to be spilled.''
BP 2GO Hokitika owner Lee Wu sympathised with the farmers, saying they must be "gutted''.
He said while the festive season had been busy for him, the last three days have been "like winter time, with no customers at all''.
"It's been really quiet,'' Mr Wu said.
"With the bridge at Harihari out, and Arthur's Pass having been closed for a wee while, we're cut off in the middle of nowhere.''
Petrol stocks were fine, he said, with terminals in Christchurch and Nelson, able to bring supplies either by Arthur's or Lewis passes.
Supermarkets in Greymouth and Westport said they'd had no problems with supplies.
New World Hokitika owner-operator, Gary Lee praised truck drivers for going the extra mile in getting foodstuffs to the store.
"There were a few delays but the drivers did an incredible job and we're now ticking along as normal.''
Motel Association West Coast president Bryan Williamson said today that "everyone was hurting a little bit''.
Mr Williamson, who owns the Bella Vista Motel, said bookings had been lost since the northern approach to the bridge was washed away.
"It has taken a lot of people off the road up here and we have lost quite a few prebookings,'' he said.
Mr Williamson said moteliers up and down the Coast were in the same boat and there was "nothing we can do, until the bridge is repaired''.
The washout could not have come at a worse time, with January the peak time for tourists from around the world visiting the West Coast: "but that's just the way it is''.
Greymouth Seaside Top 10 owner Matt Levien said they too had received a number of cancellations, but there was "no point dwelling on it''.
Tourists were having to change their plans.
"It is a nuisance, however life goes on it is no point getting upset about it,'' Mr Levien said.
