The latest leak was detected at the plant about 7am last Thursday.

ExxonMobil Australia spokeswoman Rebecca Arnold said staff immediately activated emergency response procedures.

"There was no risk to the public, nor to the environment from this incident. Mobil is disappointed the incident occurred and we are sorry for the concern that it caused within the community."

Ms Arnold said there would be a full investigation into the cause of the gas leak. "We will take appropriate action to prevent a recurrence."

The leak happened less than a month after 30,000litres of petrol spilled from a tank at the refinery.