NOPSEMA's initial findings in 2012 found that the two workers, Peter Meddens from Darwin and Barry Denholm from Lossiemouth in Scotland, died after they were struck by a manual break out tong being used to free a stuck drill pipe string.

However, the authority has acknowledged that more needs to come out of this tragedy and this week revealed that legal action will cause further delays.

"Legal proceedings have commenced through the Magistrates' Court of Victoria with the assistance of the Commonwealth Director of Public Prosecutions for specific breaches of the Offshore Petroleum and Greenhouse Gas Storage Act 2006," NOPSEMA said on Tuesday.

"Upon conclusion of the legal proceedings, NOPSEMA will communicate lessons learnt with duty holders and the wider community."

Maritime Union of Australia secretary Paddy Crumlin said two years was far too long for the families of those killed to wait for answers.

"Our highest priority is to ensure workers go home the same way they arrive - alive and healthy," he said.

"The MUA has long held concerns that NOPSEMA does not adequately work with unions to prevent safety breaches.

"This needs to change and both NOPSEMA and the Abbott government must commit to involving unions in workplace safety matters."

Australian Council of Trade Unions assistant secretary Michael Borowick was concerned NOPSEMA was "too close to the industry to act as an effective regulator".

"The fact that there has been no prosecution and the report into the investigation has not been released two years after these tragic and avoidable deaths gives weight to this concern," he said.

"Without a full and independent investigation into the tragedy we cannot ensure other workers will not be exposed to similar dangers."

Australian Workers' Union national secretary Scott McDine said to end up two years on from a severe tragedy with no action on safety reform was simply not good enough.

Rig operator Stena Drilling was working for Origin Energy in VIC/P43 in the Otway Basin, 90km south of Warrnambool in Victoria, when the accident on the semi-submersible rig occurred on August 27 2012.

NOPSEMA has previously said the drill pipe string became "stuck in the hole" on August 24.

An electrical fire occurred on the Stena Clyde rig on August 29 with only minor damage reported.