There's potential environmental disaster after a cargo ship stranded off the coast of New Zealand North Island of Tauranga could break up, leaking more oil into the ocean, according to New Zealand authorities. Maritime New Zealand said Thursday night more options are being considered to deal with an oil slick from a container ship grounded off the coast of New Zealand North Island port of Tauranga after a dispersant was ineffective, Radio New Zealand reported on Friday. The 236-meter Liberian-flagged ship, Rena, hit Astrolabe Reef on Wednesday morning, causing a five-kilometer oil slick along the Bay of Plenty coastline, already killing four birds. A 5km-long slick of oil has streamed from the 47,000 tonne Rena which struck the Astrolabe Reef about 12 nautical miles off the coast about early Wednesday morning. The 21-year-old vessel was heading toward Tauranga from Napier carrying about 1700 tonnes of fuel. Among its cargo are four containers of the hazardous substance ferro-silicon. The oil slick has streamed northwards from the ship and fuel is leaking from the vessel intermittently.

Maritime New Zealand said despite initial indications that a dispersant was working, further analysis confirmed it was ineffective.

Other dispersant options are being considered, as well as a recovery operation in which oil would be scooped off the water by teams on vessels using specialist equipment.

Maritime New Zealand believes the vessel's fuel tanks are still intact and heavy fuel oil leaking from the vessel appears to be from pipes.

The northwest direction of the oil slick poses a threat to the Department of Conservation marine and birdlife reserve Mayor Island.

Wildlife response team members have spent the night on Motiti Island, with beach searches to begin on Friday.

The reef - about 4 nautical miles north of Motiti Island and 12 nautical miles off the coast - is home to animals such as little blue penguins, seals and petrels.

Transport Minister Steven Joyce said there's a risk the ship will break up and a spokeswoman for the minister told NZ Newswire the situation is probably going to get worse.

Maritime New Zealand's National On Scene Commander Rob Service said the leak is difficult to stop due to "considerable damage" to the ship. The ship's owners have appointed international salvage company Svitzer to manage the situation, but Maritime New Zealand still has the power to take control of the operation if it is deemed necessary.