Fertiliser companies importing so-called "blood phosphate" from Western Sahara could face illegal strike action by port workers in future, warn unions.
A ship chartered by Ravensdown, the Federal Crimson, is currently unloading the last of 51 tonnes of phosphate at Port Chalmers, after earlier docking in Napier and Lyttleton.
At each port, it has been boarded by members of the Rail and Maritime Transport Union, who have handed protest letters to the captain.
Ravensdown and Ballance Agri-Nutrients import about $30 million worth of phosphate each year from Western Sahara.
Western Sahara is a piece of desert land of a similar size to New Zealand. It was a Spanish colony but was taken over by Morocco in 1975, a move condemned by the United Nations.
Union organiser John Kerr said he was "hopeful" the companies would find alternative sources - but if they did not, union members could take more drastic action next time.
"It is actually illegal in New Zealand to strike for political reasons - that right was removed by the 1991 Employment Contracts Act and not reinstated by the 1999 Employment Relations Act - but it is a human right."
Port workers who refused to allow nuclear power ships to dock in the 1980s were "pivotal" in getting New Zealand declared nuclear free, he said.
"New Zealand workers have a strong tradition of taking political action, for instance against Apartheid in South Africa and nuclear power.
"If those laws had been in place in the 1980s, workers would not have been able to withdraw their labour.
"So while this is about Western Sahara, there's a wider human rights issue too."
Last month a Western Sahara government official said New Zealand fertiliser companies are stockpiling phosphate amid threats of court action to stop future imports.
The companies say their imports have not changed.
Kamal Fadel is the Australia and New Zealand representative for Polisario, the UN-recognised independence movement for Western Sahara.
He visited New Zealand to spread awareness of the issues in Western Sahara and the impact New Zealand's phosphate imports were having on the region.
He said the Western Sahara government was close to initiating court action.
Previously, Western Sahara has successfully won in international courts, with those courts ruling Morocco had no legal claim to the phosphate, so could not sell it.
Mr Fadel said the Moroccan occupation of Western Sahara was internationally disagreed with and New Zealand's purchase of phosphate from them, was at odds with this.
Ravensdown and Ballance Agri-Nutrients are the only two importers of phosphate in the western world.
Officials at the Ministry of Foreign Affairs and Trade met with Mr Fadel during his visit and a spokesperson said afterwards that the government remained of the view that New Zealand companies should be looking for alternatives to phosphate.