International and Australian authorities are investigating a missing South Australian woman and her travelling companion who were sailing off the coast of Africa.

Port Lincoln woman Deirdre Sibly and her travelling companion, Frenchman Pascal Mahe, were sailing to Durban, South Africa from Mozambique when a distress call was sent out from the boat on Thursday.

Ms Sibly’s sister, Sue Good, said she believed the two were dead.

Ms Good said a friend of Ms Sibly’s in Port Lincoln had been informed by French Coast guards that a cargo ship had received a distress call from the yacht on Thursday and alerted local authorities. 

A woman sits in a captain's chair on a yacht, steering with her feet.

Sue Good says her sister Deirdre Sibly (pictured) was a “free spirit” who loved the sea and sailing. (Supplied: Sue Good)

“On Friday morning the Department of Foreign Affairs and Trade (DFAT) called me to say that somebody had eventually been able to board the yacht because it was in full sail, just cruising along,” Ms Good said.

“Some container ships and cargo boats had been following it since the distress signal went out and tried to communicate with the yacht, but there was no radio contact.”

Ms Good said she believed a smaller vessel launched from a maxi yacht was able to go on board, and that she was able to track the boats’ movements through a ship tracking app called MarineTraffic.

“The cargo ship, container ship and maxi yacht were all visible on MarineTraffic. They were the three boats staying in the vicinity of the yacht,” she said.

Ms Good said DFAT told her two people were found deceased on board, but no formal identification had been made, and the two were identified as a male and female.

She believed them to be Ms Sibly and Mr Mahe.

Ms Good said she had kept in contact with her sister, and the pair had left Mozambique and was sailing to Durban.

Two women stand next to a van.

Sue Good with her sister Deirdre Sibly. (Supplied: Sue Good)

“If the weather conditions were right, they were going to go to Cape Town and Deirdre was going to fly home from there.”

A DFAT spokesperson said it was providing consular assistance to the family of an Australian missing in the Mozambique channel.

“Our thoughts are with them at this distressing time,” they said.

“Owing to our privacy obligations we are unable to provide further comment.”

Ms Good said DFAT had been providing regular updates.

A life of adventure

Ms Good said Ms Sibly was an adventurous person whose life revolved around the sea and sailing. 

“She would have been sailing for about 40 years,” she said.

“She was just a lovely, friendly, kind-hearted person. She lived life to the fullest,” Ms Good said.

“We’re just devastated; she’d been away sailing with Pascal since early June … she was so happy.”