Police in Mauritius are preparing to board a grounded ship leaking tonnes of oil into its crystal-clear waters, as clean-up crews confronted a growing ecological disaster on the archipelago's pristine shores.

The bulk carrier MV Wakashio has been seeping fuel into a protected marine park boasting unspoiled coral reefs, mangrove forests and endangered species, prompting Prime Minister Pravind Jugnauth to declare an environmental emergency and appeal for international help.

Attempts to stabilise the stricken vessel, which ran aground on 25 July but only started leaking oil this week, and pump 4,000 tonnes of fuel from its hold have failed, and local authorities fear rough seas could further rupture the tanker.

Hundreds of volunteers, many smeared head-to-toe in black sludge, are marshalling along the coastline, stringing together miles of makeshift cordons in a desperate attempt to hold back the oily tide. 

But already, thick muck has inundated the island nation's unspoiled lagoons, marine habitats and white-sand beaches, causing unprecedented damage to the fragile coastal ecosystem upon which Mauritius and its economy relies.

"People have realised that they need to take things into their hands. We are here to protect our fauna and flora," said Ashok Subron, an environmental activist at Mahebourg, one of the worst-hit areas.

Police said they would execute a search warrant granted by a Mauritius court to board the Wakashio and seize items of interest, including the ship's log book and communication exchanges, as part of its investigation into the accident.

The ship's captain, a 58-year-old Indian national, will accompany officers on the search, police officials said.

Twenty crew members evacuated safely from the Japanese-owned but Panamanian-flagged ship when it ran aground are under surveillance.

Prime Minister Jugnauth thanked France for dispatching a naval vessel, military aircraft and technical experts from nearby Reunion, a French Indian Ocean island, to assist with the disaster.

He has convened an crisis committee later Sunday, and has expressed concern that forecast bad weather could further complicate efforts to stymie the spill, and cause more structural damage to the vessel.

The opposition has called for the resignation of the country's environment and fisheries ministers, as pressure mounts on the government to explain why more was not done to prepare in the two weeks since the ship ran aground.

Mitsui OSK Lines, which operates the vessel owned by another Japanese company, said efforts to place containment booms near the ship had been unsuccessful due to high waves. Helicopters had been shifting some of the 3,800 tonnes of fuel and 2,000 tonnes of diesel to land.

Aerial images show the enormous scale of the damage, with huge stretches of azure seas around the marooned cargo ship stained a deep inky black, and the region's fabled lagoons and inlets clouded over.

The Wakashio struck a reef at Pointe d'Esny, an ecological jewel fringed by idyllic beaches, colourful reefs, sanctuaries for rare and endemic wildlife, and unique RAMSAR-listed wetlands.

Mauritius and its 1.3 million inhabitants depend crucially on its seas for food and for ecotourism, and has fostered a reputation as a conservation success story and a world-class destination for nature lovers.

Ecologists fear if the ship further breaks it could inflict potentially catastrophic damage on the island nation's coastline, which forms the backbone of the economy.