Ports of Auckland says New Zealand's largest exporter Fonterra is shifting its export shipments to other ports due to ongoing strike action.
It says about $27 million worth of trade will be re-routed through Tauranga and Napier from the end of January.
Chief executive Tony Gibson says with further strike action by the Maritime Union, customers will inevitably look for alternatives.
The company and the union have been locked in a long-running dispute over the terms of a collective contract.
Mr Gibson says, following Fonterra's decision, Ports of Auckland has offered the union a 10% pay rise on hourly rates, plus performance bonuses and entitlements in return for a new roster system.
The port will keep its terminal operations open through the next scheduled industrial action on Monday, he says.
But Maritime Union president Garry Parsloe says strike action may not be to blame for Fonterra's decision.
"Fonterra shifted out of New Plymouth because it got a cheaper box rate up in Auckland; it's (now) down in Tauranga. Shipping companies do this all the time.
"I've heard Ports of Auckland say it's because of the strike action - I haven't heard Fonterra say it."
Mr Parsloe says when Danish company Maersk left Ports of Auckland it was for geographic and commercial reasons - not because of the industrial action.
In a statement issued on Wednesday, Fonterra says the decision to change ports was made to ensure certainty of supply for its international customers.