Lyttelton Port workers are threatening a three week strike over failed pay and safety negotiations.
The Rail and Maritime Transport Union has had 21 meetings with the port - the biggest in the South Island - regarding the plan to make significant roster changes without having a fatigue management policy in place.
But union organiser John Kerr said negotiations have collapsed and the union's 200 workers are prepared to go on strike and stop working overtime if the company does not return to the negotiating table.
A strike notice could be issued in the next 24 hours, he said.
The port's chief executive, Peter Davie, said he was disappointed to hear of the proposed industrial action as he believed jobs at the port were well-paid and safety was a top priority.