The murders of a young mother and her daughter remain a mystery but police say they are homing in on several suspects.

Among them, according to media reports, is a convicted pedophile serving a four-and-a-half-year sentence in NSW's Cessnock Prison.

Police are reluctant to comment on speculation about suspects but say the investigation into the death of Karlie Pearce-Stevenson and her young daughter Khandalyce is moving quickly and detectives are hopeful of a swift arrest.

Karlie is believed to have been killed some time around December 2008, in or near the Belanglo State Forest where her body was discovered in 2010.

Her daughter's remains were discovered alongside a South Australian highway in July this year but the question of where or when she died remains a mystery.