Federal police were searching a ship that docked at Port Kembla yesterday after claims a crew member was murdered at sea.
International Transport Federation co-ordinator Dean Summers said a Filipino crewman went missing from the Sage Sagittarius last weekend in the Timor Sea.
The ship had been due to load in Newcastle next week but was diverted by authorities and docked in Port Kembla about 10am yesterday.
Mr Summers said the ship's owners had flown private security guards and company representatives to the ship by helicopter from Brisbane.
Until then, most of the 21-man crew had apparently been "holed up in a cabin, fearing for their safety".
Australian Federal Police yesterday confirmed they were investigating the incident.
Mr Summers said the ship had "done a search pattern" to look for the missing seaman and alerted the Australian authorities when it couldn't find him.
"It's been suggested to us that the man met his death through foul play because he was going to come to us with an allegation of poor pay and conditions on the ship," Mr Summers said.
Mr Summers travelled to Wollongong from Sydney to see if he could gain access to the ship.
"We couldn't get on the ship because it is crawling with federal police and customs, and sniffer dogs and all sorts of things," Mr Summers said.
"It was a very busy scene but we don't want to go on board and interrupt their investigations.
"We were told by the federal police that once they're finished they'll give us a call and we can go and talk to the crew."
Mr Summers said South Coast MUA secretary Garry Keane would board the ship once police gave the all-clear.
Mr Summers also was demanding a full investigation into the incident.
"Too often these things go unreported, it happens far too often, seafarers go over the side and it's not investigated. We want to get on board to get access to the seafarers and make sure they are OK."
Mr Summers said the crew members, part of affiliated unions, would receive the support they required, including counselling.
"We will not interfere with the police investigation but we are determined to protect these seafarers and make sure they get home if that's what's needed."
The federation is an internationally recognised trade union body that fights for seafarers' rights and which works to expose so-called "ships of shame".
Mr Summers said the Sage Sagittarius was a "flag of convenience" vessel registered in Panama and owned by Japanese interests. It was a regular visitor to Newcastle.
"Unfortunately these types of events are a common occurrence on flag-of-convenience vessels; a lot of jurisdictions come into play," he said.
"But this is a ship in Australian waters and we hope the full force of the law is applied."