Two hundred workers at Lyttelton Port are refusing to work overtime as they fight for an increase in their wages.
The port is offering a 2 percent pay rise but the workers said that was not enough and pointed to the 18 percent increase the port's chief executive, Peter Davie, got this year.
The industrial action is being taken by the Rail and Maritime Transport Union, which represents half of the port workforce.
South Island organiser John Kerr said his members want the company to do a survey to get a better idea of market rates for tradespeople.
He said the port's rates fell well short of what was being paid elsewhere in Christchurch.
Another demand is for an industrial council to be set up that includes worker involvement, to address safety concerns at the port.
Three workers have died there in the past year shifting or unloading cargo.