The unions say they have sought legal advice from two senior counsel, both of whom share the view that the government had acted beyond its powers.
A MUA spokesman said the advice suggested the "government doesn't have the powers to remove such significant parts of economic activity from the migration zone.
"We are advised that there does not appear to be an act of parliament that provides for it," he said.
Industry groups have hit back at the unions' move, saying they are putting their self-interest ahead of the community by embarking on legal action intended to disrupt the vital contribution international specialists make to Australia's $A200 billion offshore oil and gas industry.
Two weeks ago, the Australian government suffered a challenge to the regulations it put in place to make the Migration Amendment (Offshore Resources Activity) Act 2013 operable.
That act came into effect on July 1.
Labor had introduced the Act in the dying days of its government and although it was approved in the Senate, regulations were not put in place to make it operate.
Without those regulations, there would be no way those who were not Australian permanent residents would be able to work in the country's offshore resources industry.
The government chose to apply the 457 and 400 class visas, which covered most non-resident workers on offshore resources facilities. It also added the maritime crew visa to cover those people that crew the specialised offshore construction vessels used in the industry.
These vessels only operate in Australia's exclusive economic zone for a matter of weeks at a time.
Without the MCVs for those few crew needed to operate the vessels, the vessel operators would be forced to take on the lengthy, costly process to get 457 visas.
The regulations were challenged by a motion put by Greens Senator Penny Wright. Her motion was passed and the regulations struck out.
For a short time these non-Australian resident offshore resources workers were in limbo.
Immigration and Border Protection Assistant Minister Michaelia Cash ended the impasse by reinstating the visa regulations as a legislative instrument.
AMOU president Wayne Moore said the Senate had made the right decision to protect Australian jobs by blocking the government's attempts to flood the offshore oil and gas sector with cheap foreign labour.
"The Australian people know that opening the back door to cheap foreign labour isn't the answer," he said.
"We need to maintain our maritime skills base and ensure the viability of Australian jobs in the offshore sector."
Australian Mines and Metals Association executive director Scott Barklamb said union claims about protecting Australian jobs were a complete fabrication.
"Through its self-interested and misleading campaign against non-Australian workers, the MUA and AMOU continue to put at risk an industry that employs 70,000 Australians," Barklamb said.
"It is very concerning to see the maritime unions mount a legal challenge against the government's restoration of offshore migration arrangements that are globally consistent and maintain established Australian regulation.
"This is a deliberate attempt to misrepresent how the offshore resource sector works and an attempt to hold the government to ransom."
Barklamb said the participation of international specialists in offshore resources activities created jobs for Australians.
"This self-interested campaign simply comes down to the maritime unions wanting to extend their control into areas where they have no historic legal right or practical reason to do so," he said.
