Unions say a Hong Kong-based ship berthed in Port Kembla has a crew which has not been paid since June, and is owed $100,000.
The International Transport Workers' Federation (ITF) has called on the Australian Maritime Safety Authority to investigate after seafarers on the MV Xing Ning Hai reported they hadn't been paid in months.
The carrier, owned by Chinese company Dalian Ocean Prosperity, is in Port Kembla to carry steel from BlueScope.
Dalian's MV Xing Jing Hai was detained by AMSA in Brisbane on Wednesday following almost-identical allegations of unpaid wages. It is carrying clinker for Cement Australia.
Between the two there was more than $200,000 owed in wages, ITF assistant co-ordinator Matt Purcell said.
"These cases show that massive wage theft and the exploitation of vulnerable foreign seafarers are not an anomaly, they are a central feature of the business models of many of the shipping operators carrying freight to and from Australia," he said.
"All these vessels have been contracted to carry goods and materials for major businesses.
"AMSA deserves to be commended for acting swiftly once issues are identified, but the current system relies on the efforts of ITF inspectors and whistle-blowers among ship crews to identify problems, meaning countless cases of exploitation are slipping through the gaps."
The MV Xing Ning Hai was last year detained by AMSA in Tasmania, after an ITF inspection found the crew had not been paid for six months, the union said. More than $300,000 was recovered in unpaid wages before it was allowed to depart Australian waters.
The Mercury has contacted Dalian Ocean Prosperity and AMSA seeking comment.
ITF president Paddy Crumlin said the cases highlighted the urgent need for stronger shipping laws and more proactive enforcement to counter the growing use of highly-exploited foreign workers in Australian waters.
"In recent decades, the number of Australian-flagged vessels has been slashed, with local seafarers replaced by exploited foreign crews on ships registered in notorious tax havens," Mr Crumlin said.
"The country is now almost entirely dependent on foreign flag of convenience vessels, often registered in tax havens and crewed by exploited workers on as little as $2 per hour.
"What these three incidents show is that this isn't an occasional issue involving one rogue operator, it's a central part of the business model of a growing number of companies that are contracted to supply Australia's fuel, carry our resources, and move cargo to and from our port."