A $1 million reward has been offered by police for information about the murders of three Aboriginal children in Bowraville, on the NSW Mid North Coast.

Key points: A prime suspect in the case has been acquitted and had a retrial refused, and cannot face a new trial due to double jeopardy rules

A prime suspect in the case has been acquitted and had a retrial refused, and cannot face a new trial due to double jeopardy rules Police have increased the reward from $250,000 to $1 million in a bid to uncover fresh information

Police have increased the reward from $250,000 to $1 million in a bid to uncover fresh information Three children were murdered in Bowraville in the early 1990s

Four-year-old Evelyn Greenup and 16-year-olds Clinton Speedy-Duroux and Colleen Walker went missing in a five-month period between 1990 and 1991.

The prime suspect — who can't be named for legal reasons — has previously been acquitted of the murders of Clinton and Evelyn.

The children's families today said they were "absolutely gutted" at the Attorney-General's "slap in the face" revelation that the State Government will not back law reform to double jeopardy rules, which would have paved the way for a new trial.

Evelyn's aunt, Michelle Jarrett, said it left them with no recourse to achieve justice for the children.

"It's unbelievable. I'm gutted, I'm heartbroken," she said.

"I've lost all faith in NSW Police, the politicians of NSW and our justice system."

The NSW Government last year lost a last-ditch effort to have the matter considered by the High Court due to new evidence about the death of Colleen Walker.

An undated photograph of Colleen Walker, whose body was never found. ( AAP: Supplied )

The Criminal Court of Appeal had earlier held that evidence related to Colleen Walker's death was previously "available" at the trial of Evelyn — even if inadmissible — and therefore couldn't be considered fresh evidence.

One of two proposed new models would have allowed unlimited applications for a retrial.

But NSW Attorney-General Mark Speakman defended his decision, saying the double jeopardy rule was important and had been part of the system for 800 years.

"This is not about keeping guilty people out of jail, this is about protecting the innocent when they have the full might of the state against them," he said.

"It's also making sure we have finality in criminal litigation — that these things are brought to a conclusion."

Mr Speakman said he couldn't, in good faith, "drag" the families through an application for retrial when he saw it as not having realistic prospects of success.

NSW Police Commissioner Mick Fuller urged those with any knowledge of the matters to think hard about what they remember from the early '90s.

"Any single piece of information in relation to this case could help homicide detectives solve it," he said.

The head of the homicide squad, Detective Superintendent Danny Doherty, stressed the investigation remains open.

"It's ongoing and we'll never give up on this investigation," he said.