Industrial action is starting at Ports of Auckland on Thursday because of a dispute with the Maritime Union about its collective agreement.
The port's container terminals will close for four days from 10.30pm.
The union had announced the withdrawal of more than 320 of the 580 employees involved in container handling and road and rail services at Waitemata seaport for two 24 hour periods.
Ports of Auckland issued a lockout notice in response for two days following the strike, closing the container terminals until 5 December.
Chief executive Tony Gibson says the port is working closely with KiwiRail and the Port of Tauranga to minimise disruption, but an estimated $1.2 million in revenue will be lost.
He says jobs could go if strike action continues.
The Auckland Chamber of Commerce says the planned industrial action is already starting to cost businesses and disrupt pre-Christmas planning.
Its head, Michael Barnett, says going on strike just before Christmas is holding the port's business customers to ransom.
