The Maritime Union says it is taking legal action after being unable to talk to the crew of the Manahau, which ran aground on the West Coast almost two weeks ago - with 11 foreign seafarers on board.
Meanwhile another union, the Merchant Service Guild says it is concerned about reports the crew's employer is preventing welfare checks aboard the barge - as checking on foreign crew was essential to ensure New Zealand's supply chain was free of trafficked labour.
The Manahau has been detained in Nelson, while investigations by the Transport Accident Investigation Commission, Maritime New Zealand and the operator WMS Group are underway into the cause of the 31 August grouding on Carters Beach, near Westport.
Maritime Union of New Zealand national secretary Carl Findlay said its representatives had been blocked from visiting the Manahau since its arrival in Nelson and WMS Group were still trying to prevent its crew from accessing support.
But a spokesperson for WMS Group said two senior Maritime Union New Zealand officials boarded the Manahau on Thursday morning, spoke with the crew and inspected the conditions on board.
The company said it also organised a meeting with union officials and legal representatives at the Rutherford Hotel in Nelson, but none of the crew took up the option to join the union.
But the Maritime Union said it had not invited the Manahau's crew to join the organisation, it was still trying to ascertain their employment conditions and immigration status.
"Our support for them is as overseas seafarers in a difficult situation, from countries including Myanmar with a very poor record around the rights of seafarers."
It maintained that it was facing difficulties when trying to talk to the crew onboard the stricken vessel.
WMS Group said the crew all hold visas that permitted them to work in New Zealand, were on individual employment agreements governed by New Zealand law and were paid high market rates.
It said the welfare of the Manahau's crew remained its highest priority and it was doing everything it could to ensure they were cared for and safe as it worked with the authorities on the investigation.
Findlay said the pre-arranged meeting on Thursday between the union's lawyer, officials and the crew was interrupted by a company representative who drove off with crew members.
"We are extremely concerned about the attitude of WMS, the company who operates the Manahau, and its approach to the rights of crew and legitimate public interest in this debacle."
The union said it found overseas seafarers could not always be free and frank with their employers around, and they came from jurisdictions where they could get into very serious trouble by standing up for their rights.
The Maritime Union is affiliated with the International Transport Workers' Federation (ITF) which represents over one million seafarers in more than 200 seafarers' unions across 106 countries. Maritime Union officials are empowered to act as ITF inspectors aboard ships.
"Crew members should have independent support and advice, as the company that put them on this vessel can't be trusted to do the right thing."
A Port Nelson spokesperson said as part of its obligations under the Maritime Security Act 2004 and as a Customs controlled area, anyone visiting a vessel must be verified with the visit approved by the vessel's representative.
On Wednesday, Maritime Union staff were restricted from entering the port operational area and visiting the vessel as they had no approved purpose or agreement from the vessel owner.
The following day, union personnel with ITF accreditation were allowed access to the vessel.
The spokesperson said other authorised parties had also accessed the vessel to perform welfare checks.
New Zealand Merchant Service Guild vice president Captain Iain MacLeod said allowing foreign crew access to welfare workers, like ITF representatives was essential to protect vulnerable international crew from abuse, exploitation and human trafficking.
Captain MacLeod said that drove John Key's 2010 Ministerial Inquiry into migrant labour abuses unearthed by the Auckland School of Business on deep sea fishing vessels working in New Zealand.
He said despite receiving $7 million in government funding for the Manahau, the NZ operator's plan from day one was to employ migrant labour on the vessel.
Westland Mineral Sands Group managing director Ray Mudgway quoted in the Westport News saying; "WMS planned to employ between 14 and 18 international crew because the vessel would be working around the clock."
MacLeod said he was unsure why that was relevant as it was routine for New Zealand ships' crews to operate 24/7 while on board a vessel.