Coastal shipping will be under pressure to cope if South Island roads fail to hold up under extra traffic created by the Kaikōura earthquake, KiwiRail says.
Traffic has been heavy on State Highway 63 south of Blenheim, which is the new main route between Picton and Christchurch.
KiwiRail chief executive Peter Reidy said the company was keeping an eye on State Highways 63 and 7, which he said would peak at about 1000 trucks a day this week - five times the usual freight traffic.
KiwiRail moves about 1 million tonnes of freight per year between Picton and Christchurch, the equivalent of 80,000 trucks.
Mr Reidy's main concern was domestic freight, which he said was a large and growing part of KiwiRail's business.
"The key supply line now is constrained and we are worried if the road doesn't hold up, because that will mean that all that volume will have to go via coastal [shipping]."
There was coastal shipping capacity, but KiwiRail's domestic customers needed to be sure they would get freight, such as groceries, within 48 hours, Mr Reidy said.
"There is capacity but it's the reliability of that capacity, it's the guaranteed times [of] departure [and] arrival."
The company had organised coastal shipping between Auckland and Christchurch three times a week and was exploring other options, such as chartering a foreign vessel, he said.
KiwiRail had put a freight container terminal in place in Blenheim to help its customers transfer containers from rail wagons onto trucks, and ferries would be running to full capacity from today.