That followed the union's refusal to collaborate in a "continuous improvement" project needed to make Auckland's waterfront competitive with rivals such Tauranga's port.
Mr Gibson said the loss of about 12 per cent of the port's container business, worth about $2 million annually, to Tauranga in a decision last week by Danish shipping giant Maersk, also meant possible redundancies.
"My clear message to the union from the outset is that you either change with us or change happens around you, and we would prefer they were at the table," he told the Herald. "We don't write intimidatory letters to our staff and we never will - it's all about being honest and transparent."
In one of the latest letters sent to Mr Parsloe and workers, Mr Gibson said the company intended to review the desirability of contracting out "some or all of its container terminal operations and support services to third-party contractors." That was in addition to possible redundancies from the Maersk decision.
Mr Parsloe said he had set time aside for another attempt to resolve the dispute on Thursday and Friday this week, but was unable to lift notices of two more strikes before and on Christmas Day while the company was upsetting members so much.