On the 28th anniversary of the events which led to Penny's death, the NSW Government announced the increase on Monday morning and re-released photos and information on the case.

Penny Hill, then aged 20, was found unconscious with severe head and facial injuries on Cassilis Road, near Coolah, about 8am on Monday, July 8, 1991.

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She was taken to John Hunter Hospital, where she died two weeks later, on Sunday, July 21, 1991.

In 1992, a coronial inquest held in Mudgee returned an opening finding on the cause and manner of death.

Detectives from the State Crime Command's Homicide Squad commenced further investigations into Penny's murder under Strike Force Samdon, before a second coronial inquest was held in 2012.

The second inquest also delivered an opening finding and was returned to investigators, who pursued a number of new lines of inquiry, however, no one has ever been charged in relation to Penny's death.

NSW Police Homicide Squad Commander Detective Acting Superintendent Mark Henney said the increased reward is for information which leads to the arrest and conviction of those responsible for Penny's murder.

He said almost 30 years of searching for answers, the Hill family deserve closure.

"Penny had only just moved to Coolah - to begin her first job as a nanny at the Black Stump Motel - just three days before her body was found," he said.

"Detectives have made a number of public appeals to the community for any information that could help investigators solve this horrific murder.

"Over the years, a number of leads have been pursued and today's reward is another opportunity to appeal to those people in the community who have information about Penny's death to come forward.

"We are committed to getting justice for Penny and providing answers for her family."

Penny's mother, Jeanette Hill, said the last 28 years have caused unbearable pain.

"There is simply no way to describe the immense agony of losing a child," she said.

"Every day I am reminded of Penny, of the tragic way in which she was taken from us, and of the life she never got to live.

"It has been almost three decades that we have been searching for answers and once again we ask that anyone in the community who might know what happened to our daughter, please come forward and help police.

"Our family, and our Penny need peace."

Anyone with information that may assist Strike Force Samdon investigators is urged to contact Crime Stoppers on 1800 333 000 or visit 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.