Then aged 20, Penny was found unconscious with severe head and facial injuries on Cassilis Road, near Coolah, a town in NSW's Central West, about 8am on Monday July 8, 1991.

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

In 2019, on the 29th anniversary of her death, the reward for information into her murder was increased to $1 million - the offer still remains.

A new Nine Network program, Million Dollar Murders, premiering on Monday May 23 will look at unsolved cases around Australia where rewards for information have been set at $1 million. Episode two, set to air on May 30, puts the focus on the unsolved murder in Coolah.

The Mudgee Guardian spoke with Bryan Cockerill, Executive Producer on the show.

Bryan and his team have been working on the show for a number of years, contending with COVID lockdowns that delayed their visit to Coolah to conduct interviews and research for the program.

He said one of the most challenging aspects of investigating a small-town murder is that everyone has a theory.

"That's always the hardest thing, especially in these in these kinds of cases too that people think they're being very helpful when they pass on every bit of gossip they hear," Bryan said.

"It does mean that, of course, the police then have to run all of these leads down.

"I think that's always a tricky thing. But in this poor case with Penny was the problem that from the start [it was treated] as an assault. And so it was just investigated by local police. It wasn't until three weeks later that she actually died, and by that stage there was no evidence left in the room. Tragically, some evidence was disposed off, which may have proved crucial, but no one will ever know.

"So it's one of those terrible situations where police actually had to start very much on the backfoot because they really didn't get a chance to do proper investigating until three weeks after the assault had happened."

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Bryan hopes this renewed attention on the case can lead to closure for Penny's family.

"People have got - it's very difficult in these circumstances for people to keep these secrets. And that's kind of the thing too, because probably they will have to have told someone or someone would have to know something...," he said.

The show will air on the Nine Network on Monday May 23 at 9pm on the Nine Network and the Coolah episode will air one week after on May 30.

Host and retired Detective Superintendent Deborah Wallace worked alongside some of the country's leading detectives. Wallace questioned witnesses and spoke with grieving family members.

In 1992, a coronial inquest held in Mudgee, in NSW's Central West, returned an opening finding on the cause and manner of death of Penny Hill.

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.