Orange County California supervisor Katrina Foley released a statement saying an estimated 570,000 litres of the material produced an oil slick covering some 30sq.km of the Pacific Ocean since the leak was first reported on Saturday.

The spill is believed to have originated from a subsea pipeline asset attached to the Elly rig, which is owned and operated by Amplify subsidiary Beta Operating. 

Multiple local news outlets, including CBS and the Los Angeles Times have reported the oil leaked from the Elly rig

Elly is one of three platforms operated by Amplify Energy in the offshore US Beta Field, overseen by the US Department of the Interior. 

Huntington Beach mayor Kim Carr also confirmed at a press conference that some 33sq.km had been affected

She called the spill an "environmental catastrophe" and a "potential ecological disaster".

Overnight Sunday Australian time, the oil has infiltrated an area known as the Talbert Marsh, a sizable nature reserve which is thought to have been significantly damaged.  

Supervisor Foley made one tweet referencing "dolphins swimming through" an oil slick.

The beaches were closed and a planned Pacific Air Show scheduled for the region was cancelled. 

The US Coast Guard has provided monitoring exercises to determine the extent of the incident.

Reuters reports that Amplify Energy CEO Martyn Willsher told a press conference that the pipeline had been shut off and remaining oil had been suctioned out. 

He added that divers were still trying to determine where and why the spill occurred.