The workers, 32-year-old Northern Territory toolpusher Barry Denholm and 60-year-old floorman Peter Meddens from Scotland, were killed by a "machinery failure" as they drilled for gas off the Victorian coastline, about 90km southeast of Warnambool.

Early reports indicated the men died operating heavy machinery on August 27, 2012.

The machine failed when part of a drill dislodged and struck the men, one an Origin employee and the other employed by Stena.

The National Offshore Safety and Environmental Management Authority said yesterday that Stena entered its plea in the Victorian Magistrates Court, agreeing that it had breached its duty to the men and had not reasonably taken all practicable steps to implement and maintain systems of work that were safe.

Prior to referring the matter to the Commonwealth Director of Public Prosecutions for prosecution, the National Offshore Petroleum Safety and Environmental Management Authority conducted an independent, rigorous and exhaustive investigation into the accident, consulting with the workers who were on the rig that day and experts about the equipment that was involved.

NOPSEMA submitted a brief of evidence to the DPP in April 2014.

"This prosecution has reinforced the requirement for an appropriate risk assessment system to be implemented for all stages of work," NOPSEMA CEO Stuart Smith said.

"Workers involved should have an opportunity to contribute to this assessment including consideration of factors such as stored energy; equipment design limits; and, impact of external conditions.

"Communication is a key part of any work offshore and supervisors should verify that all workers involved in any task understand their role and any associated risks.

"All equipment utilised in planned work should be fit for purpose and in good working order. If the equipment is not working correctly, a reassessment of the risks associated with the work or task should be conducted."

NOPSEMA's investigation identified that senior management on the Stena Clyde failed to apply the Stena management of change principles in failing to carry out a new risk assessment and toolbox talk after altering the original plan of works.

Further, Stena conceded that senior members of the drilling crew failed to ensure that a revised risk assessment had been carried out prior to implementing the new plan.

The Australian Workers Union conducted its own probe into the tragedy, which was a rare serious accident in what is an inherently dangerous industry.

The Australian Council of Trade Unions has called for the offshore oil and gas workers to be covered by the same OH&S laws as mainland workers.

The matter is now listed for sentencing on September 3.