The MUA and the Australian Maritime Officer's Union secured a judgement that overturned attempts by both the Turnbull and Abbott Coalition governments to overturn decades of practice and require offshore workers to have visas to operate in Australian waters.

The case, which dates back to Abbott's era, has its roots in a move by former Assistant Immigration Minister and now Employment Minister Michaelia Cash to use an obscure ‘legislative instrument' to "ignore the will of the Senate", as the unions put it, that had disallowed government moves to remove offshore visa requirements from the migration zone.

The unions saw Cash's ministerial ruling as an audacious move prompted by what they believed was the Abbott government's ideological war on unions.

Armed with legal advice the MUA and AMOU launched a High Court challenged and after more than a year emerged victorious yesterday with all five judges determining that Cash's move to change the definition of offshore resources activity was invalid.

The unions were also awarded costs.

Deputy national secretary Will Tracey said it was a great result that confirmed Australian workers' rights to work in the offshore space.

"The High Court has backed in the right for seafarers today and should also send a signal to the Australian government, Michaelia Cash and Peter Dutton in particular, that they should listen to the will of the Senate, the courts and the people," Tracey said.

"Up until this point in time, industry had the ability to pay visa workers on anything they like, which in the domestic blue industry has been $2 an hour..

"If we had lost this, in 12 to 18 months there wouldn't be any Australian seafarers working in the industry.

MUA national secretary Paddy Crumlin said he always was optimistic the court would "see through the blatant skullduggery and legislative trickery the Abbott/Turnbull government had used to ignore the Senate and the Federal Court".

"Unions watched the matter very closely because it was a blatant attack on Australians' rights to work in their own country," Crumlin said.

"The Turnbull government's use of a cynical and flawed legislative process undermines certainty and investment confidence in Australia's essential hydrocarbon industry."

Offshore workers will now be required to meet Australian immigration requirements and will need to go through the process of getting a 400 or 457 visa.

Minister for Immigration and Border Protection Peter Dutton said the over-turning of Cash's decision was "disappointing".

"Many of these vessels operate in international waters and never enter an Australian port," he said.

He said the limited visa exemption had protected and supported Australian jobs, but that had been thrown away "at the behest of the militant Maritime Union".

"This will add red tape, add costs to industry and reduce the competitiveness of what is one of Australia's biggest export earners," Dutton said.

He justified the government's move to excise the visas from the migration zone by quoting a 2012 a Federal Court judgement found that workers on these types of vessels were not in Australia's migration zone and not required to hold visas.

The case will have major implications for the $200 billion offshore construction sector which accounts for some $28 billion of economic activity.

Employee advocate group the Australian Mines and Metals Association said the ruling would require some projects and some service providers to urgently obtain visas before non-Australian nationals can recommence work.

AMMA executive director Scott Barklamb said that, as most of Australia's offshore resource projects have moved beyond peak construction, the immediate impact of the decision was not as pressing or widespread as it would have been two years ago.

However, the group is still concerned about the longer-term cumulative impact of additional regulatory costs on Australia's competitiveness as a resource economy.

"The role of non-Australian crew members on highly specialised construction vessels is a very small, but critical part of building offshore resource projects," Barklamb said.

"Making the employment of such workers more regulated, more costly and more difficult in Australia than it is in competing resource economies will have consequences and will make it even harder to bring future resource investment to Australia.

"Australia needs to be doing all it can to position itself to attract the next wave of global resources investment, and today's judgement means Australia now regulates work supporting the offshore resources sector differently to the rest of the world."

The immigration department has already updated its advice for foreign workers in or preparing to enter Australia, telling them to secure the appropriate visas as soon as possible.