South African airways said it was investigating the involvement of a syndicate in thefts from sleeping passengers aboard international flights, and has already blacklisted one passenger.

The airline’s statement came after a passenger reported that a fellow traveller had stolen jewellery and about $1,800 dollars in foreign currency from his luggage in the overhead locker on a flight from Johannesburg to Hong Kong.

"We are now aware of three incidents of suspected pilferage and theft on board our aircraft that occurred within three weeks of each other on route between Johannesburg and Hong Kong," a SAA spokesman said.

"The incidents occurred within a short space of time on one route to give rise to suspicion that a syndicate could be involved."

Passenger Warren Becker described in an email to Traveller24 his experience on board a flight from Johannesburg to Hong Kong.

"I had just woken up from a sleep when a passenger seated a few rows behind me advised me to check my bag, as she had seen some other passengers take bags from the overhead lockers and were rummaging around in them near the bathrooms," he wrote.

Mr Becker said he alerted flight attendants, one of whom told him that this kind of theft had happened before "and there was some kind of 'syndicate' operating on SAA planes and that 'I had a right to know about this'".

Local media said the man suspected of going through Mr Becker's baggage, as well one of his supposed accomplices, were searched by Hong Kong police officers, but the money was not found on them.

"We plead with our passengers to take precaution with their personal belongings and be vigilant during the flight," the SAA spokesman said.

"We are extremely concerned that we have experienced such unfortunate events that leave a bad after taste on our passengers' flight experience.