"The reward for information into the murder of Evelyn Greenup, Clinton-Speedy Duroux and Colleen Walker-Craig from Bowraville have each increased to $1m," NSW Police announced in a tweet on Friday.
The three children, ranging in ages from four to 16, disappeared from the northern NSW town over a five-month period from September 1990.
The announcement comes nearly two years after a previous $1 million reward yielded no breakthrough in the cold case.
The murders were originally investigated separately before being linked by the homicide squad.
A man, who can't be named for legal reasons, was acquitted of Clinton's murder in 1994 and of Evelyn's murder in 2006.
The government in 2018 unsuccessfully argued in the NSW Court of Criminal Appeal that there was fresh and compelling evidence - related to the disappearance of Colleen - which justified overturning the two acquittals and a new single trial on three murder charges.
The High Court refused to grant special leave to the NSW government to appeal the decision.
Greens MP David Shoebridge in 2019 lobbied to change the state's double-jeopardy laws after the High Court decision.
Under the laws revised in 2006, a person can only be tried for the same crime for which they've previously been acquitted if there's fresh and compelling evidence.
Mr Shoebridge sought to amend the law to facilitate a retrial of the suspected killer in the Bowraville murders.
Yet, a NSW upper house inquiry found the bill would impact on too many unrelated parts of the criminal justice system and should not proceed.
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