Last week NSW Police announced the reward for information about the cold case had been increased from $100,000 to $1 million.
A man and woman, both 61, were arrested at a property at the town of Harden about 7.30am on Wednesday.
They were taken to Young Police Station, where police say they are expected to be charged with murder.
Ms Haigh was reported missing on June 19, 2002, after she failed 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Later that night money was withdrawn from her bank account at a Campbelltown ATM.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.
Last week, strike force detectives travelled to the Albury suburb of Thurgoona and seized a white Suzuki Sierra for further forensic examination.
A video message from Ms Haigh's heartbroken mother Rosalind Wright was released last week.
"I know in my heart that she would never have left her son," Ms Wright said.
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