Marion Bay holidaymakers stumbled across suspected cocaine while on a beach walk

Stunned Marion Bay holidaymakers on a beachside walk stumbled across a bag which is suspected to be packed with cocaine worth millions of dollars.

Holiday-makers walking along the beach at Marion Bay on Sunday stumbled across a black bag that police suspect is full of cocaine.

The bag has been seized for forensic examination.

The discovery comes after a shipment of $250m worth of cocaine was seized by police off Yorke Peninsula in an operation dubbed Operation Lithgow, just over two weeks ago.

Police at Marion Bay after holidaymakers found a suspected block of cocaine in a bag washed up on the beach.

The Australian Federal Police, Australian Border Force and SA Police joint press conference last month to announce the state's biggest-ever cocaine bust. Picture: Tom Huntley

The seizure was the largest of any type of drug in South Australian history and had been the focus of an intensive air, sea and land search by officers after intelligence was received.

A 225m-long grain carrier had dumped the drugs in the water, several nautical miles off the tip of Yorke Peninsula, at what police believed was a predetermined location.

Police found the shipment and seized the drugs with the help of more than 60 Australian Border Force officers who were involved in searching for the drop-off site.

The vessel came to SA from Ninghbo, China.

Four Filipino men have been charged over the cocaine bust and have faced court.

The discovery was just under half of the total amount of cocaine seized across Australia last year.