Ravensdown Fertiliser says it is working to stave off possible action against the cargo ship Federal Crimson, headed to the Ports of Napier, Lyttelton and then on to Dunedin.
The ship is carrying 51,000 tonnes of phosphate rock sourced in Western Sahara, and advocacy group Take Em Down Otautahi has pledged a "peaceful blockade" of the ship.
It is due to arrive in Napier on December 1, departing for Lyttelton on December 5.
The planned action is supported by local maritime unions, following last month's resolution by the international Council of Trade Unions condemning Morocco's illegal occupation of Western Sahara and to halt importation of phosphates from the area into New Zealand.
Rail and Maritime Transport Union South Island organiser John Kerr said the union made a request to Ravensdown for its members to board the vessel and deliver a letter of protest, but was declined.
"We believe that an orderly, peaceful protest in this manner would appease those committed to more radical, disruptive tactics".
Kerr said fellow unionists in Australia, the Maritime Union of Australia, had already used this tactic to register protests against phosphate imports.
"To us it seems a relatively mild and pragmatic way to exercise a fundamental democratic right without risking undue disruption.
"Unfortunately, Ravensdown appear to prefer confrontation to dialogue" he said.
Ravensdown spokesman Gareth Richards said it remained open to the idea of discussing the issue with stakeholders and had initiated discussion with the unions, though any intended protest action was beyond its control.
"Ravensdown acknowledges the right to protest, despite our perspectives being different. For example, the trade is legal, complies with UN expectations and welcomed by the Saharawi who are employed at the Phosboucraa mine".
Richards confirmed that the company "typically" imported three ships a year of rock from the area and this had not changed.
"Our policy is to encourage the UN to take all efforts towards a political solution of the dispute, do what we can to explore additional sources of phosphate rock and continue to encourage OCP to do what it can for the local people".