Supermarket shelves in the earthquake-stricken region of North Canterbury will remain stocked if the Ports of Auckland has any say in it.

Chief executive Tony Gibson said freight demand on New Zealand's busiest domestic sea route, Auckland to Christchurch, had doubled following the Kaikoura earthquake on Monday.

Gibson said Ports of Auckland would be "pulling out all the stops to ensure that people in Christchurch and affected areas in the Upper South Island have continuous access to the things they need".

"While the sudden increase in freight does put pressure on us, we will do everything we can to make sure the goods get through as quickly as possible," Gibson said.

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He said the company was working closely with Pacifica, New Zealand's domestic coastal shipping operator, and participating international lines to make sure the freight gets through now parts of the region were cut off by road and rail.

"South Island goods are our top priority. It is only a small thing, but we are pleased to be able to help where we can," he added.

Pacifica chief executive Steve Chapman said the supply chain was open and the shelves would remain well stocked as coastal shipping took up the task previously carried out by road and rail.

"With the main road and rail links likely to be closed for months, we expect to be run off our feet, but we are well up to the challenge."

Pacifica's coastal container ship Spirit of Canterbury would be in Auckland loading containers for Lyttelton on Thursday morning.

There are five weekly services between Auckland and Lyttelton operated by domestic and international lines, with ample capacity to serve the current level of demand and more, Chapman said.