The Maritime Union is questioning the legitimacy of a new union for waterside workers in Auckland which this week signed a collective agreement with the port company.
The Portpro Union began bargaining for a new contract on 19 September and reached a settlement less than a week later.
In a statement, Ports of Auckland thanked the new union for a positive and constructive bargaining approach which it said was without disruption.
Maritime Union president Gary Parsloe said the new union must have reached an agreement on the company's terms.
After months of strikes, lockouts and protests, the Maritime Union and Ports of Auckland go back into facilitation on 15 October.
The Maritime Union has 300 members at the port.