NSW Police have sent divers to search Sydney Harbour after a new lead emerged in the four-decade-old assassination of a Turkish diplomat and his bodyguard.

Sarik Ariyak, 50, and his bodyguard Engin Sever, 28, were fatally gunned down outside a home in Dover Heights on December 17, 1980.

The pair were leaving the Portland Street residence in separate vehicles when they were approached by two unknown men who fired several shots at close range before fleeing on a motorcycle.

Police say information received after the reward was bumped up prompted the harbour search. Photo: AAP

Mr Ariyak died at the scene and Mr Sever died a short time later at St Vincent’s Hospital.

Despite extensive investigations by NSW Police at the time and in the years since, no one has ever been charged.

After a formal review by the NSW Joint Counter Terrorism Team, police divers conducted searches off Greenwich Point “regarding items of interest to the investigation”.

With a $1 million reward for information on offer, the JCTT has appealed for public help after new details relating to the cold-case double-execution came to light.

NSW Police Counter Terrorism and Special Tactics Commander, Assistant Commissioner Mark Walton said when the reward for information was increased to $1 million on the 39th anniversary of the deaths in December, it led to new lines of inquiry.

Subsequently police divers from Marine Area Command recently conducted searches in Sydney Harbour off Greenwich Point.

As investigations continue – in Australia and overseas – detectives are keen to speak with anyone who might know who was responsible for planning, carrying out or covering up the crime.

“Operation Esslemont investigators have been working meticulously through the original case files and have established new and significant lines of inquiry,” Assistant Commissioner Walton said on Thursday.

“Since our appeal late last year, we have received incredible interest from the community but given the politically motivated nature of the assassination, we believe there’s still a reluctance to help us.

“But experience tells us that the passage of time – particularly changes in relationships between people with knowledge of a crime – coupled with a hefty reward can be the key to breaking the wall of silence,” he said.

-with AAP