The pair, both aged 61, were arrested at a property in the southwest NSW town of Harden at about 7.30am on Wednesday, and have since been charged with one count each of murder.

The man was also charged with aggravated sexual assault of a victim with serious intellectual disability, NSW Police said in a statement.

Both were refused bail and will face court on Thursday.

Ms Haigh was reported missing on June 19, 2002 after failing to return to her home at Kingsvale, a town in the South West Slopes, where she and her six-month-old son had been living with a married couple.

Police were told the couple dropped the young mother at Campbelltown station on June 5 and she intended to travel by train to Mt Druitt in western Sydney to visit her sick father in hospital.

Later that night, money was withdrawn from her bank account at a Campbelltown ATM.

A coronial inquest in 2011 found Ms Haigh died as a result of homicide or other misadventure in June 2002.

In 2020, a formal review of the case was conducted under the Homicide Squad's Unsolved Homicide framework and Strike Force Villamar II began a fresh investigation.

NSW Police last week announced the reward for information about the cold case had been increased from $100,000 to $1 million.

A video message from Ms Haigh's heartbroken mother Rosalind Wright was also released.

"I know in my heart that she would never have left her son," she said in the video.

Australian Associated Press