Victoria's gas supply is under threat as workers at the Qenos petrochemical plant in Melbourne's west prepare to strike for a fifth day in a row, over an enterprise bargaining dispute.

One of the state's biggest gas producers, Esso, has told the Victorian government to prepare for a potential reduction in gas supply, due to ongoing industrial action.

On Monday morning, 35 Qenos plant operators and technicians walked off the job, demanding better pay and conditions.

Qenos, in Altona, is Australia's largest plastics producer and uses ethane extracted from the Esso Long Island Point gas plant to make its products.

"Without an outlet for the ethane, Esso will need to significantly curtail natural gas supply to reduce the production of these gas liquids," Esso said in a statement.

"Esso has informed the Australian Energy Market Operator (AEMO) as well as the Victorian Department of Environment, Land, Water and Planning so that both are aware of, and can plan for, the potential reduction in supply from Longford Plants, including looking to other supplies to fill any shortfall", Esso said in a statement.

A government spokesperson said while it was monitoring the situation at the Qenos plant, there was no impact on gas supply.

"AEMO has advised that there is no current disruption to gas supply, including the use for electricity generation," the spokesperson said.

However, the government said the dispute had the potential to impact production at the Longford Gas plant if left unresolved and that it would continue to monitor the situation.

Qenos has applied to the Fair Work Commission to terminate the protected industrial action.

"I think it is a real shame Qenos [is] seeking to hide behind the law instead of negotiating in good faith," Australian Workers Union Secretary Ben Davis said.

"Qenos were quick to let the guys work through COVID lockdowns and were happy for them to generate revenue for the company, so now it is time to return the favour, but instead it's ignored 90 per cent of [their] claims."

Workers complain of unsafe conditions

One worker — who did not want to be named — told the ABC their main concern was the safe staffing of the plant, with the closure of a second Qenos plant last year meaning more work was being done with fewer staff.

In a statement, Qenos said it had issued workers with an "indefinite lockout" in response to the industrial action.

"The resins plant was safely shutdown and is now under a safety watch, the olefins plant has reduced [its] ethane intake and ethylene production, and Qenos continues to work closely with its feedstock suppliers on operational contingencies," the statement said.

"Qenos will continue to review all available options to address the operational impact of the protected industrial action."

Last year, Qenos shut two of its refineries in Altona, at a cost of around 150 jobs.

The company blamed the closure of ExxonMobil's Altona refinery for restricting the amount of LPG available to feed its own plastics production.