Van Silfhout said the Council of Trade Unions (CTU) passed a resolution condemning Morocco's illegal occupation of Western Sahara and calling on the New Zealand Government to halt the importation of phosphates from the area in 2019.
"As an affiliate to the CTU the RMTU registered our protest by meeting the captain of the vessel in the early hours of this morning and handing him a letter that included the CTU resolution."
Due to Covid-19 restrictions, the letter was delivered "without direct contact" between protesters and the crew, she said.
Van Silfhout acknowledged there had been "constructive dialogues" with Ravensdown.
"Whilst we fundamentally disagree about the importation of blood phosphate they have been reasonable in facilitating a lawful and safe protest, as have Napier Port management and the shipping agent."
A Napier Port spokesperson said the port supported their RMTU branch being able to deliver a letter of protest in the same safe and peaceful manner as has been done previously by the RMTU.
Ravensdown spokesman Gareth Richards said Napier Port was a "vital part" of the supply chain, bringing in essential inputs such as fertiliser that local farmers and growers could turn into outputs like food to export.
"Thousands of local jobs are dependent on the moving and processing of these inputs and outputs including at the port itself."
He said shipping and logistics of essential nutrients like phosphate rock had become even more important in a world rocked by Covid-19, soaring costs and lower availability.
"Ravensdown's advice continues to be the trade is legal, local people in Western Sahara are helped by the trade and the UN is best suited to arbitrate between the competing claims in a complex geopolitical dispute."
Napier Port has been approached for comment.