More than 200 members of the Maritime Union of New Zealand and the Rail and Maritime Transport Union at Port Chalmers voted unanimously for the ban after rejecting a Port Otago pay offer of 6% this year and 4% next year.
This is the first industrial action in about 21 years at Port Chalmers.
The main point of contention between the parties is agreement on the hours worked on the third graveyard shift and its starting time to unload container vessels, of which there has been a 70% increase in volume during the past three years.
Port Otago said it had implemented a plan to unload seven container ships this week without the need for overtime, causing minimal disruption for the ships, but the union has said the overtime ban would have a cumulative effect toward the end of the week.
There is no date set for negotiations to resume, to replace the three-year contract which expired in February with a new two-year contract.
