A Singapore-flagged cargo ship which lost more than 50 containers off the NSW coastline was rolling and veering forward in rough seas when it spilled part of its load overboard, a preliminary Australian investigation has found.
The APL England, which was en route from China to Melbourne, dropped the containers southeast of Sydney on May 24 after a temporary loss of propulsion.
The ship's cargo, including face masks and plastic food containers, littered NSW beaches for months after the incident and prompted massive clean-up efforts across 45 beaches.
An Australian Transport Safety Bureau preliminary investigation found the APL England was rolling and "pitching", or veering forwards and backwards, amid high seas and gale-force winds when the ship's main engine shut down due to a loss of oil pressure.
The crew restarted the engine and had turned to travel north when it noticed some 37 minutes after the incident that its stacks of containers had fallen.
The APL England reached the Port of Brisbane, where the ATSB found many of the ship's fittings required for holding containers in place had corroded and wasted.
"It was later established that 50 containers had been lost overboard from both forward and aft bays," ATSB chief commissioner Greg Hood said in a statement on Wednesday.
"One container lost overboard contained hazardous goods in the form of dry powder fire extinguishers, while 23 containers were empty.
"A further 63 containers were damaged but remained on board."
Mr Hood said no safety findings or analysis had yet been established.
The APL England left Brisbane in June to return to China for repairs, having made a written promise to pay fines and other agreed or court-mandated amounts.
The ship's 43-year-old captain, Mohamad Zulkhaili Bin Alias, was also in June granted permission to return home to Malaysia.
He has been charged with two offences relating to the discharge of garbage into the sea and failing to ensure a vessel is operated so as not to cause pollution or damage to the Australian marine environment.
The bill for the incident was estimated as high as $22.5 million.
Mr Hood said the ATSB would review several areas of interest including the ship's maintenance regimes, service and inspection history, and the actions of the crew.