Rewards of $100,000 are being offered for information leading to a conviction in two historic suspected murder cases.

Tanya Farrington and Linda Suzanne Davie both disappeared in suspicious circumstances more than 30 years ago and are presumed by police to have been murdered.

NSW Detective Sergeant Robert George responded to media reports that police were investigating possible links between the girls and notorious criminals including underworld identity John Anderson, serial killer Ivan Milat and New Zealander "Mr Asia" drug syndicate head Terry Clark.

"Any suggestion of those links is just speculation," he told reporters in Sydney on Sunday.

Tanya was 14 when she went missing from her Crows Nest home on Sydney's north shore on March 22, 1979, he said.

She was last seen by her brother watching TV late at night in the family home but was gone when her mother woke up in the morning.

Ms Davie, then 22, has not been seen since she returned to her north shore Wollstonecraft home after visiting her boyfriend in Royal North Shore Hospital on April 6, 1980.

The New Zealander's boyfriend received a letter saying that she would be away until the following week - she was never seen again.

Neither of the women had any reason to deliberately disappear, Det Sgt George said.

"They have not contacted their families in over 30 years," he said.

"We presume that what has happened to them is a result of murder or foul play."

NSW Police Minister Michael Daley said Tanya often snuck out at night and may have hitchhiked to Manly to spend time with friends.

"I'd urge anyone who saw Tanya that night, or might have information about her disappearance to come forward," he told reporters.

Police would also like to speak to Tanya's boyfriend at the time and two men who are believed to have driven Ms Davie and a friend home from Kings Cross on April 5, 1980.

Police are asking anyone with information to call crime stoppers on 1800 333 000.