The Maritime Union of New Zealand (Munz) and Port Otago began pay talks in late February to on a two-year contract, replacing a three-year contract which expired on February 28, but talks and mediation had failed.
Munz national president and Port Chalmers' branch secretary Phil Adams said 200 union members voted unanimously on Tuesday in favour of an overtime ban beginning on July 2.
Mediation between the parties was scheduled for next week.
He did not rule out seeking another vote for increasing the level of action if talks failed to break the impasse.
While pay rates were part of the problem, changes to an established third shift at night to unload container ships had become the major sticking point.
Port Otago chief executive Geoff Plunket said yesterday he was "disappointed" with the notice of strike action, which followed rejection of the company's latest offer, including an immediate 6% pay rise, followed by a further 4% rise a year later.
"With the growth in container volumes of more than 70% during the past three years, it is becoming increasingly important for Port Otago to be able to operate three container cranes 24 hours per day, when required, in order to provide the level of service expected by international shipping lines," he said in a statement.
Mr Adams said workers had been doing three shifts at Port Chalmers for about nine years.
The company now wanted a 10-hour third shift and to be able to use three cranes at a time, to service the large 4100-container vessels which arrived on Thursdays and Fridays.
Port Otago also wanted staff to begin shifts when a vessel docked, as opposed to the present system of staff working within an established roster of three eight-hour shifts.
Port Otago had grown so quickly it did not have staff numbers to accommodate all the ship calls adequately, which was placing pressure on existing staff - about 210, plus management, he said.
Mr Plunket said as part of any settlement, Port Otago wanted changes to allow for three-crane operation, when required, over 24 hours, and shift arrangements which could handle increasing volumes.
