The protesters were opposed to the arrival of Amoy Dream, which earlier docked at Ravendown's Ravensbourne wharf, and which was also met by protesters in Lyttelton on Sunday.
The ship carries fertiliser from Western Sahara, a disputed territory in North Africa which Morocco is accused of illegally occupying.
Environmental Justice Otepoti member Sam Murphy was one of the protesters who described the ship's contents as "blood phosphate".
He said it carried phosphate "stolen" from Western Sahara.
Police arrived at the protest but the demonstrators quickly moved up towards State Highway 88 and the truck was free to head north.
Ravensdown spokesman Gareth Richards earlier said the Moroccan state-owned OCP company had assured Ravensdown all funds from the phosphate mine were invested in local programmes that benefited the local Sahrawi people.
"It remains our position that we are trading legally, operating within UN expectations," Mr Richards said.