IT is a murder mystery buried beneath tonnes of sand for more than three decades.

But the sands of time may soon give up their grisly secret with revelations human bones found at the site belong to a man and a woman.

The Daily Telegraph can reveal forensic tests carried out on the bones, dug up during construction of the new desalination plant in Kurnell - near Botany Bay in Sydney's south - show they are the remains of two people who died less than 50 years ago.

And it is understood that rubbish found with some of the bones date them to more recent times - from the 1970s onwards.

This has raised speculation they may belong to one of a number of couples who vanished from New South Wales in the late 1970s and early 1980s.

Carbon dating tests on the bones have been conducted in New Zealand and the bones are being sent to the US for extraction of DNA.

There is still no clue as to whether the pair were murdered, died accidentally, or even died together.

However, the dating of the bones has ruled out the prospect they were ancient Aboriginal bones, as was originally suspected due to their location at Kurnell.

The discovery of the bones was first made in early October when workers digging in sand dunes unearthed a shin bone. The following week, 300m away, ribs and other, smaller bones were discovered nearby in sandy scrubland off Sir Joseph Banks Drive.

This was followed by the unearthing of a pelvis and foot bones - still wearing a sock - nearby.

A forensic pathologist and an anthropologist from the University of Sydney were called to the scene to examine the bones and police conducted excavation of the area and sent in cadaver dogs.

The Daily Telegraph understands that police are skeptical the remains could be victims of jailed underworld figure Neddy Smith, whose alleged dumping ground was Botany.

It is understood that while police have conducted an initial check of missing persons records, the search criteria is seen as too wide.

However, there are three high-profile cases of missing couples dating back 30 years that remain unsolved.

Michelle Pope, 18, and her boyfriend Stephen Lapthorne, 21, went missing in August 1978 after leaving his north-west Sydney home in a green van.

Alan Fox, 21, and his fiancee Anneke Adriaansen, 19, have been formally declared dead by a coroner after disappearing during a trip to Byron Bay in January 1979.

Joanne Lacey and Leslie Toshack, both 20, were reported missing to police in early 1981 and had planned to hitchhike up the coast.

The Kurnell bones will be tested in the US for mitochondrial DNA - a form of DNA which lasts longer than nuclear DNA in bones and runs in the maternal line.

In the past two years, a DNA testing program has led to relatives of 84 people who had gone missing as far back as the 1960s, supplying DNA. This DNA is for comparison to almost 150 sets of unidentified remains.

Police said that so far this program had resulted in six missing people being matched to remains.

The Kurnell site is also a short distance from the still-unsolved Wanda Beach murders.

Best friends Christine Sharrock and Marianne Schmidt, both aged 15, went missing from Cronulla's Wanda Beach in January 1965 and their bodies were found in the sand the following day.