(CNN) -- A major clean-up operation is underway along the north coast of New Zealand 's North Island as debris and oil leaking from a cargo ship that ran aground on a reef wash ashore, officials say.
The Rena, a Liberian-flagged vessel, struck the Astrolabe Reef, about 12 nautical miles off the city of Tauranga, last week.
New Zealand's oil spill response agency, Maritime New Zealand (MNZ), estimates that as much as 300 tonnes of fuel oil has leaked from the vessel, which was carrying 1,700 cubic meters (450,000 gallons) of fuel.
The country's environment minister, Nick Smith, has called the spill the country's most significant maritime environmental disaster.
As of Wednesday, clean-up teams, comprised of some 500 responders, collected 50 tonnes of solid waste and five tonnes of liquid waste from beaches in the Bay of Plenty, cleaning oil from 17 kilometers of coastline, MNZ said.
Pubic access to some waterfront areas has been restricted due to health concerns.
MNZ said the coastline between the towns of Whangamata and Whakatane will be assessed today to determine the areas of highest priority for clean-up operations.
"There is a massive effort underway and it is only going to grow," said National On Scene Commander Nick Quinn. "We understand the anger and disbelief we're hearing from people," he said.
"Our focus is on recovering oil from wherever we find it and we will go in day by day until this is over."
MNZ said that 200 dead birds have been recovered following the oil spill and that this number is expected to increase "significantly."
The agency said 36 wildlife rescue teams are scouring the coast for wildlife affected by the spill and 47 oiled animals are being treated at center that has been set up in Tauranga. A team has been established to capture seals and five of the animals are being kept in captivity.
Greenpeace has expressed "extreme concern" about the spill and urged the government to avoid using further toxic dispersants.
"This is an unfortunate illustration of just how difficult it is to deal with oil spills at sea," the organization's Steve Abel said.
"Even a slow and relatively accessible oil spill like this one has clearly stretched New Zealand's response capability to its limits," he said.
"It is also a potential disaster for the blue whales and dolphins presently calving in the area, as well as numerous other marine species."
MNZ said that about 70 containers had fallen off the ship, some of which have washed ashore. Due to the hazard the containers pose to shipping, the agency said navigational warnings had been issued and major maritime traffic has been re-routed.
The Rena has suffered substantial structural failure and there is a concern that the stern of the vessel may break up, MNZ said.
Salvage teams have three tugs mobilized either to hold the stern on the reef while efforts continue to remove oil from the ship, or to tow the stern to shallow water to extract the oil, the agency said.
Containers remaining on the vessel continue to move, making it dangerous for salvage crews to work on board.
The ship's second officer will appear in Tauranga District Court this morning to face a charge of "operating a vessel in a manner causing unnecessary danger or risk," MNZ said.
The Rena's captain was yesterday remanded on bail on the same charge, on the condition he surrender his passport. His name has not been released.
If convicted, they face a maximum fine of $7,800, or up to 12 months imprisonment.
CNN's Karen Smith and Laura Smith-Spark contributed to this report.