New South Wales Police are investigating the suspicious death of a crewman on board one of the world's largest coal carrying ships which docked in Newcastle yesterday.

It is the second mysterious death on the ship, the Sage Sagittarius, in as many weeks.

In the first incident a crew member disappeared off the ship while it was sailing north of Cairns in the Timor Sea.

Today, ambulance officers worked in vain hoping to save the second Filipino crewman's life shortly after the ship docked.

When they failed and the man died, police sealed the ship off and declared it a crime scene.

They are remaining tightlipped about what happened.

Glen Williams, the local branch secretary with the Maritime Union of Australia, says he believes the man in his 50s was the chief engineer.

"He'd been on board the vessel for I would imagine somewhere near seven, eight, nine months, as these guys do," Mr Williams said.

"He was due to leave the vessel tomorrow to go home, and obviously his family was expecting him.

"So that's the tragedy in all this; there's a second crew member in just over a week who won't be going home to his family."

Police say the ship is now a crime scene and they are investigating the death of the crew member.

The Sage Sagittarius had run into crew trouble a fortnight ago when another Filipino seafarer disappeared off the ship in the Timor Sea, 400 nautical miles north of Cairns.

Shortly after the disappearance the rest of the crew barricaded themselves in a secure part of the ship and were said to be terrified.

The Maritime Union said the man may have been preparing to complain about his working conditions.

Authorities ordered the ship berth at Port Kembla rather than the intended destination of Newcastle while they attempted to open an investigation into that incident.

Uncertainty

Mr Williams says he is not sure if the second crew member was alive when the ship finally docked in Newcastle.

"All I know is that he was very seriously injured," he said.

"I have heard a couple of things today that a possible heart attack and a fall resulting from that, but we just don't know at the moment.

"Nobody's releasing any statements as far as we're aware and there's really no information coming from the police at the moment."

The ship is 11 years old and flies under the flag of Panama.

It is operated by the giant Japanese shipping company Nippon Yusen Kaisha and managed by Hachiuma Steamship.

Both companies declined PM's request for an interview.

In a statement though, Hachiuma Steamship said the company is working closely with authorities and it expresses its heartfelt condolences to the family of the dead crew member.

It also says the company will provide support to his family in the Philippines.

Issues on board

Mr Williams says even before the deaths, the Sage Sagittarius was an unhappy ship.

"The crew did have concerns in regards to the wages and conditions, which is not unusual on these ships," he said.

"We get a lot of foreign ships coming into our country that are treated poorly, that have issues with unpaid wages, poor supplies of food and water on board the vessels.

"So I understand that was an issue prior to the crew member disappearing a week or so ago.

"So our main concern at the moment is for the welfare of the crew. "

The Maritime Union says it would also like to help the grieving crew members in Newcastle but so far they have not been given access to the ship.