Protesters have rallied outside NSW Parliament House to demand justice for three Aboriginal children killed in northern NSW more than 20 years ago.

About 100 people marched from Hyde Park to state parliament chanting "justice, justice" on Thursday and calling for Attorney-General Greg Smith to reopen the unsolved murder cases.

Colleen Walker, 16, Clinton Speedy, 16, and four-year-old Evelyn Greenup all disappeared from the Bowraville mission over a six month period in 1990.

"We haven't moved on and we won't rest until we get justice," Ronella Jerome, Clinton's aunt, told AAP.

"We've been asking for answers for 23 years.

"Until we get justice and the man responsible for these deaths is in jail where he belongs, our families will continue to walk around like lost souls, angry and frustrated."

The protesters said the initial police investigation into the disappearance of the three victims was flawed, as highlighted by a subsequent coroners inquest.

AAP