Ports of Auckland, New Zealand's largest container port, said yesterday's mediation with the Maritime Union was unsuccessful and it is preparing for a strike next month that will see some ships diverted to Tauranga.

Ports of Auckland chief executive Tony Gibson said the Bledisloe and Fergusson container terminals, and associated road and rail services would be closed from 10.30 pm on December 1 until 10.30 pm on December 5. The disruptions would cost about $1.6 million.

"This stoppage is especially concerning given that we are now in the lead up to Christmas, one of the busiest times in the shipping schedule," he said.

The port is talking to customers to minimise disruptions but at this stage four ships were likely to be diverted to rival Port of Tauranga, New Zealand's biggest export hub, and two could miss Auckland from their shipping schedule entirely, Gibson said.

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"The total number of containers affected is likely to be between 6000 and 7000," he said.

The mediation was an attempt to reach agreement on the terms of a new collective pay contract for waterfront workers. The strike involves 327 of the ports 500 employees.