Vic schoolgirl murder reward to be $1m

The reward over the "Mr Cruel" murder of 13-year-old Karmein Chan is set to rise to $1 million, 25 years after the Melbourne schoolgirl's abduction.

Police are expected to increase the $100,000 reward about Wednesday's anniversary of her abduction from her Templestowe home on April 13, 1991.

The man dubbed Mr Cruel is believed to have been responsible for at least four child kidnappings and sexual assaults in Melbourne in the late 1980s and early '90s.

Two other girls kidnapped by Mr Cruel were released but Karmein's remains were found a year after her abduction. She had been shot in the head.

The original task force investigated more than 10,000 pieces of information and a second investigative team reviewed its work and new leads, exhausting certain avenues of inquiry and eliminating several potential suspects, between 2010 and 2013.

Police have said the Mr Cruel cases will never be closed until they are solved.

New rewards for unsolved Victorian murders have been set at $1 million since December 2014, with past reward cases being reviewed with a view to having their amounts increased.

Police say an announcement regarding an increased reward in the Karmein Chan murder will be made soon.