Exxon Mobil Corp. said it would keep its Cobia oil platform in the Bass Strait closed while it investigates the incident.
Cobia operations are overseen by a joint venture between Exxon and BHP Billiton. BHP Billiton had no comment available about the spill. The Australian newspaper reported Tuesday about five barrels of oil was estimated to have spilled from the platform last week.
Exxon said sheen was observed near the base of the platform but it has dispersed naturally.
"An investigation is under way as to the cause of the incident and we regret that this incident has occurred," Exxon said Monday.
The newspaper Tuesday said Cobia is among a series of platforms working to develop one of the largest oil and gas regions in the country. The platform is about 30 years old.
Exxon said its total production of energy in Australia is expected to increase at an average rate of 2.9 percent through 2030.