However, they believe crucial evidence needed to catch those who executed Consul-General Sarik Ariyak and his bodyguard Engin Sever remains with someone willing to finally divulge secrets they've held for 40 years.

The NSW Joint Counter Terrorism Team, which includes Australian Federal Police and the Australian Security Intelligence Organisation, has renewed its investigation into the killings.

Mr Ariyak, 50, and 28-year-old Mr Sever were shot multiple times at close range by two men when leaving the consul-general's Dover Heights home in separate cars at 9.45am on December 17.

The attackers fled on a motorcycle, leaving Mr Ariyak to die in front of his 12-year-old daughter. Mr Sever died a short time later at St Vincent's Hospital.

Soon after, the Justice Commandos of the Armenian Genocide claimed responsibility, but no one has been charged despite extensive investigations.

The JCTT set up Operation Esslemont last year to re-investigate the murders using advancements in investigative techniques and forensic technologies.

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.

Those led police divers to carry out underwater searches on Wednesday in Sydney Harbour off Greenwich Point, where they recovered some items.

But detectives say they won't be able to solve the crime without new information.

"It's like a jigsaw puzzle with frayed edges. We're trying to put it together and every piece of information that we have brought to us actually helps us put it all together," NSW Police Counter Terrorism Commander Mark Walton told reporters on Thursday.

The team believes the assassinations are linked to a string of attacks on Turkish officials around the world supposedly carried out by Armenian-linked terror organisations.

"If we look at the history of over 30 similar acts that occurred around the world over that period during the '70s and '80s ... this was consistent with that and the motive is clearly connected," Assistant Commissioner Walton said.

"It's reasonable to expect that the same people would be connected between those two crimes, as they are likely to be with many of the crimes that occurred overseas during that period."

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

"This wasn't one or two people that were involved in this activity. There would have been more involved, which I think increases the chances that ... some of them are alive and are sitting there today with that on their conscience," Mr Walton said.

Australian Associated Press