As the operation to stem the oil slick caused by the stricken Rena off New Zealand's east coast continues, salvage teams have resumed efforts to reach tanks on board Reuters

The ship spilling hundreds of tonnes oil into the ocean just off New Zealand is on the brink of breaking up after a crack appeared all the way around its hull, authorities have said.

A salvage crew has been winched aboard during a calm weather in what may be the last desperate effort to limit the country's worst maritime environmental disaster. They will assess whether the remaining oil can be pumped into ships alongside before the Rena comes apart.

A vertical crack in the ship that Maritime New Zealand described as a "substantial structural failure" runs around the entire vessel. Steve Jones, spokesman for Maritime New Zealand, said the ship was now only held together by its internal components.

"The reality is the vessel could break up at any point," Jones said. "Conditions are very calm out at the moment ... If we're going to get oil off before the ship breaks up, today's the day."

Maritime New Zealand estimates that at least 350 tonnesof heavy fuel oil have spilled from the hull. Officials believe the ship originally had about 1,900 tonnes of oil and diesel on board.

The Liberian-flagged Rena ran aground on 5 October on Astrolabe Reef, 14 miles (22km) from Tauranga harbour on New Zealand's North Island. About 88 containers have fallen off as it has listed increasingly in stormy ocean conditions.

The ship's 44-year-old Filipino captain, whose name has not been revealed publicly, has been charged with operating a vessel in a manner causing unnecessary danger or risk. He was released on bail on Wednesday from Tauranga district court. The ship's second officer is expected to face a similar charge on Thursday.

The captain's lawyer, Paul Mabey, requested that Judge Robert Wolff withhold his client's name because, he said, "there is a real potential that some persons may want to take matters into their own hands" the New Zealand Herald reported on its website, adding that the grounding occurred on the captain's birthday.

If convicted the captain could face a fine of up to NZ$10,000 and 12 months in prison. His next court appearance is 19 October when authorities say more charges are likely.

Clumps of oil have washed up on pristine beaches near Tauranga. Maritime New Zealand said 200 oiled birds had been found dead and 47 others were being cleaned at a wildlife emergency centre.

Several miles of coastline had been closed to the public and some beaches were severely covered, Jones said. "I was down there this morning. It was just black coming in – just black, black, black."

Witnesses said dead fish were washing ashore as local volunteers with plastic gloves and buckets worked to clean the oily clots from the white sand.

In a statement the owners of the vessel, Greece-based Costamare Inc., said they were "co-operating fully with local authorities" and were making every effort to "control and minimise the environmental consequences of this incident". The company did not offer any explanation for the grounding.