NSW Police Commissioner Mick Fuller was joined by NSW Attorney General Mark Speakman to officially announce the reward increase today (Wednesday), following a briefing with the families of Colleen Walker-Craig, Evelyn Greenup and Clinton Speedy-Duroux.

Colleen Walker-Craig was aged just 16 when she was last seen at a party in Bowraville on Thursday 13 September 1990.

Her family reported her missing the following day, and her weighted down clothes were later found in the Nambucca River.

Although Colleen's body has never been found, the NSW Coroner found she had died and had most likely been murdered.

Her four-year-old cousin, Evelyn Greenup, disappeared after being put to bed at her grandmother's Bowraville home on Thursday 4 October 1990.

Her remains were found in bushland near Congarinni Road on Saturday 21 April 1991.

While the post mortem did not conclusively determine the cause of death, it was noted that a skull injury was consistent with a forceful penetration by a sharp instrument.

Clinton Speedy-Duroux was also aged 16 when he disappeared after a party at Bowraville on Thursday 31 January 1991.

His remains were located in bushland near Congarinni Road on Monday 18 February 1991.

The murders were originally investigated separately before being linked by the Homicide Squad under Strike Force Ancud, and despite a man being charged on separate occasions over two of the children's murders, he was acquitted.

Following further inquiries, Strike Force Ancud investigators took the matter to the Attorney General, who applied to the NSW Court of Criminal Appeal for a retrial in 2017.

The matter was heard at the High Court of Australia, but the application was refused.

The children's families have since been advised that the investigation will undergo a formal review by the Unsolved Homicide Unit, but fresh information will be required to complement the exhaustive inquiries previously conducted by Strike Force Ancud.

As such, the reward for information has been increased from $250,000 to $1 million.

Commissioner Fuller said the announcement of the increased reward is the next step in support of the families' ongoing fight for justice.

"The increased reward forms part of our commitment to providing justice - firstly to the victims and their families - but also to the entire Bowraville community," Commissioner Fuller said.

"I'm in awe of their strength and resilience, but I'm conscious that the sins of the past have required such resolve."

The Commissioner added that with up to $1 million up for grabs for information, he hoped this may finally lead police to an arrest and a conviction through the courts.

"Rewards can be that final motivation needed for someone to speak with police," Commissioner Fuller said.

"These murders have never left the hearts and minds of the Bowraville community, and captured the attention of people around the world, but to progress, we need people to come forward.

"I believe this reward gives us an opportunity to appeal to those who know what happened to Evelyn, Clinton and Colleen, but may have been reluctant to talk to detectives in the past.

"This is one of our most challenging investigations - but we don't back down from a challenge - we will continue to stand alongside the children's families and the Bowraville community in the pursuit of justice."

Anyone with information is urged to contact Crime Stoppers: 1800 333 000 or https://nsw.crimestoppers.com.au. Information is treated in strict confidence. The public is reminded not to report crime via NSW Police social media pages.