Oil producer ESSO has locked maintenance workers out of it plants in Victoria and the Bass Strait in response to overtime bans imposed by unions.
ABC reported the EBA negotiations between the Electrical Trades Union have reached a stalemate, with claims the company will continue to press claims considered untenable by union members.
ETU organiser Peter Mooney said the lockouts would delay maintenance schedules and could have an impact on safety.
The union had already imposed bans on overtime and call-outs, in the effort to prevent Esso from making changes to pay and conditions.
Mooney said workers had voted in secret ballots and shown overwhelming opposition to the proposed changes.
"We've had no meetings where there's been any opportunity to progress with the unions and the company," he said.
Maintenance workers will be locked out of the Longford gas plant, Long Island Point, as well as offshore plants for two hours each day.