Cash introduced the legislative instrument yesterday, which effectively winds the visa situation back to where it was pre-June 29.

It means a non-citizen will be required to hold an appropriate work visa, such as a subclass 457 visa, for offshore resources activities involving an Australian resources installation fixed to the Australian seabed, such as a traditional oil rig.

A maritime crew visa will only be valid for those working as the crew on a ship.

Other offshore resources activity would not come within the migration zone and therefore there were no visa requirements.

The union movement is understandably unhappy about the development and is weighing its legal options.

The oil and gas industry is just happy that a situation that threatened its existence has been sorted out.

The visa problem was created on Wednesday when a motion from Greens Senator Penny Wright was passed that disallowed a regulation specifying the type of visas which would be needed by those working in the offshore resources industry.

The regulation had been put in place to satisfy Labor legislation that effectively came into effect on July 1.

"The support of the Greens disallowance motion resulted in a situation where a person who is not an Australian citizen or permanent resident was in breach of their temporary visa conditions if they participated in or supported an offshore resources activity," Cash said.

"Labor senators capitulated to the ideological demands of the most militant union in Australia in a move that plunged our domestic oil and gas industry into an avoidable state of uncertainty."

That was why, Cash said, she took the legislative instrument action to rectify the situation.

Australian Mines and Metals Association executive director Scott Barklamb said Cash had found a "sensible fix to the uncertainty and potential chaos" the Opposition senators had created.

"She has issued a legislative instrument that returns the regulation of international employment in Australia's offshore resources sector to the way it operated under the previous Labor government, the way it always operated in Australia and the way it operates globally," he said.

"This is a straightforward job-supporting solution to a highly complex and damaging problem."

So how is it that a legislative instrument can undo something enshrined in an act of Parliament?

The explanation from government solicitors runs to seven points and takes up most of an A4 page.

Basically it says that the Migration Act allows the minister to take such action. It also says the legislative instrument cannot be disallowed the way the regulations were.

No doubt this legislative instrument will be scrutinised by lawyers in the coming days.

Maritime Union of Australia secretary Paddy Crumlin said the union was considering its options.

"The MUA is seeking legal advice but it appears the Abbott government is treating this week's senate decision with disdain by trying to ride roughshod over existing legislation, which protects the rights of Australian maritime workers to work in their own country," he said.