The Maritime Union of Australia (MUA) is hailing a High Court decision it says will protect local jobs and prevent the exploitation of foreign workers in the offshore oil and gas industry.

The Court has ruled that Federal Government measures to allow foreign labour on offshore facilities are invalid.

Unions had argued that the move provided incentives for companies to hire foreigners on cheaper wages and undercut safety standards.

The Government said the Court's decision was "disappointing" and would reduce the competitiveness of one of Australia's biggest export earners.

Working visa exemption opened door to foreign crews

The MUA is one of the two unions which led the challenge in the High Court, and it described the decision as a victory for Australian workers.

"There's thousands of Australian seafarers who earn their living in the Australian offshore oil and gas industry and this decision confirms their rightful place to continue to work, as they have done for many many years now," said Will Tracey, the MUA's deputy national secretary.

In December last year the Federal Government granted a working visa exemption to foreign workers on offshore 'vessels'.

But unions claimed that move was aimed at getting rid of Australian workers, and replacing them with foreign workers on far less pay and reduced safety conditions.

"Certainly I think for some of the big players it was an attempt to try and drive down costs, because the reality of this decision was it would have made available to the industry foreign labour, as much foreign labour as you wanted, and at any price," Mr Tracey said.

"So, you know, people being paid $2 an hour and all sorts of things we've seen over the last number of years."

Mr Tracey said today's decision protected the rights of both Australian and foreign workers.

"We've certainly protected the rights of Australian workers in the first instance, because there will now be a requirement that they give them first go at the work.

"But certainly this will protect the rights of foreign workers on the basis that if there is an established need for labour to be brought into the country, they're paid the same terms and conditions of employment as Australian workers."

The High Court decision now means that foreign workers on offshore projects require visas that make them subject to local pay and conditions.

Slater and Gordon, which acted as the solicitors for the MUA and Maritime Officers Union, is pleased with the decision.

"Legally, you've got the minister making a determination under an act which the High Court has found basically takes away everything that is in the act, and so it is a case of acting outside of powers, and so it's been struck down," said Kathryn Presdee, an industrial lawyer at the law firm.

"We think that if you're employed to work on the Australian oil and gas fields you should have a visa if you're not an Australian citizen or permanent resident."

Decision won't increase wages or conditions: industry body

The Coalition has made numerous times to exempt foreign workers in the offshore oil industry from visa requirements.

These attempts were first blocked by the Senate in July 2014 and then again by the full federal court in March 2015.

The Federal Government then used an obscure and rarely used "legislative instrument" to circumvent the court and the Senate.

Today the High Court struck it down, saying it was invalid.

In a statement, Immigration Minister Peter Dutton described the decision as disappointing and said it would hurt the competitiveness of the offshore resources industry.

The Australian Mines and Metals Association (AMMA), the peak body for the industry, said it would add regulatory burdens on companies undertaking offshore projects.

"Today's decision will in no way make employment in the industry safer, it will in no way increase wages or conditions for people working in the international sphere," said Scott Barklamb, the executive director of policy and public affairs at AMMA.

"In fact our understanding is that the employment concerned has been undertaken in accordance with the laws of the sea and the international maritime labour convention for many years.

"So there's a lot of misinformation being spoken and it's not appropriate to exaggerate the importance of today's decision, but rather to think about its impacts on the industry and our ability to do business globally, competitively."

AMMA said some projects and service providers would need to urgently obtain 400 or 457 visas before non-Australians could start work again.