The Greens are worried New Zealand might be breaking international laws, with the arrival of a shipment of phosphate from Western Sahara in Africa.

A cargo has been partly unloaded in Lyttelton.

But the Green Party believes it is being sold by the Moroccan state-owned mining company, but without the consent of those in Western Sahara.

Co-leader Russel Norman says the United Nations has previously said it believes the phosphate is being exploited without regard to the local people.

He says it is potentially damaging to New Zealand's reputation.