Companies have a "responsibility" to ensure their shipping supply chains are not taking part in slavery, according to maritime lawyer Glenn Kolomeitz.
The lawyer and ship surveyor based in the NSW South Coast town of Gerroa has lodged a submission with the federal government's National Action Plan to Combat Modern Slavery.
Mr Kolomeitz said flag of convenience ships are registered in countries like Panama and Liberia, where any enforcement efforts are minimal.
"Once it does that a shipping company knows it's pretty much got carte blanche to do what it wants," Mr Kolomeitz said.
And what it can do is treat seafarers quite badly; turning them into slaves who are stuck on board for more than a year at a time in what is called "debt bondage".
"A seafarer from a Third World country has to buy their job and that money is loaned to them by the seafarer's recruiter in their Third World country," Mr Kolomeitz said.
"They then go on the ship in debt having bought their job via a loan that they have to pay that back while they're on the ship.
"Their wages are usually pretty abhorrent so they don't get paid all.
"They've got no say in their destiny because they're told 'you owe us money'. You can't go onshore and you have to put up with the conditions.
"They're often stuck on ships at times that far exceed their contracts. There's a ship about to come into Melbourne and be detained - they've been on that ship 15 months."
In 2019 a BlueScope-chartered ship was at the centre of allegations of "wage theft" from an international union.
At the time BlueScope stated they were a steelmaker, not a transport company.
Mr Kolomeitz said companies like BlueScope had a responsibility under Australia's modern slavery act to ensure their supply chains were slavery-free.
He said they could "send a very powerful message".
"They aren't aren't a transport company but, yes they have obligations at law to exercise due diligence regarding slavery on ships in their supply chain," he said.
"At the coalface the companies have a corporate social responsibility to help remediate the conditions of the seafarers and return them to a better state than they are currently in."
In its first statement on modern slavery, released last month, BlueScope CEO Mark Vassella said companies could not afford to be "complacent" about the risk of being directly linked to modern slavery.
"We take this risk seriously and embrace our duty to raise awareness, lift standards and change practices where they are found to be wanting," he said.
In regards to shipping, a BlueScope spokesman said the steelmaker only chartered reputable shipping companies via RightShip, "a third party maritime operational due diligence organisation, who vets vessels and drives standards in the industry beyond compliance".
"We also do a lot on the ground, from our employees inspecting the ships arriving on our berths, checking on the welfare of the crew and providing care packages, as well as having a BlueScope representative on the Board of the Missions to Seafarers," the spokesman said.