A heavy swell has forced crews to stop pumping oil off the stricken container ship Rena, and authorities are warning more containers could fall overboard with the vessel at the mercy of the weather.
More than 350 tonnes of oil have spilled from the ship, and 88 containers have fallen overboard since it grounded on the Astrolabe Reef in New Zealand's Bay of Plenty on October 5.
Maritime New Zealand (MNZ) salvage unit head Kenny Crawford says salvors have been unable to finish a dam to access the ship's submerged starboard fuel tank, which contains an estimated 358 tonnes of heavy fuel oil.
He said they had made "great progress" in the past few days during a window of good weather, but operations would have to remain suspended until the sea conditions improved.
Five-metre swells were forecast for the next 48 hours.
Mr Crawford warned it was likely more containers would be lost overboard during the rough weather.
Salvage company Svitzer was hoping to attach tracking devices to dangerous goods containers, and others that were most likely to tumble into the sea.
Salvors spent Monday sealing the ship's fuel tanks and air vents in case conditions worsened.
"This should help limit further release of oil in the worst-case scenario of the ship breaking up," Mr Crawford said.
More than 1000 tonnes of fuel had been pumped off the Rena as of Sunday afternoon, and four of its five main fuel tanks were now empty.
But MNZ's national on scene commander Mick Courtnell warned oil was likely to continue leaking from the Rena's damaged duct keel and other pockets where it had been trapped.
"This may result in more oil coming ashore, but exactly where will depend on wind and tide conditions," he says.
The rough weather could worsen the leak, but it also may aid the natural dispersion of the oil.
Oil clean-up efforts continued on Monday on Matakana Island, Papamoa and Maketu while reports of an oil slick south of the East Cape have been confirmed as algal bloom.
