DNA evidence may help solve the murder of a young NSW nanny more than 20 years ago, with police collecting samples from more than 100 people across the state.

Penny Hill, 20, died from her injuries two weeks after she was found unconscious on the side of a country NSW road on July 8, 1991.

The young woman had just started working as a live-in nanny for former Billy Thorpe and the Aztecs drummer Col Baigent and his then wife, Barbara Baigent, at the Black Stump Motel in Coolah.

In July, a second inquest into her death failed to shed any light on how she died, and the case was referred back to homicide police.

Now, in a bid to 'follow every rabbit down a hole', police are taking DNA samples from anyone who was in the Coolah area, or had contact with Ms Hill in 1991.

Detective Sergeant Jason Darcy said every person required to give a sample has previously been questioned by police in connection with the murder.

'It's a basic process of elimination,' he told AAP on Wednesday.

'The DNA material we got ... we don't know where it fits into the investigation.'

Police will test the new DNA samples against evidence collected from the scene at the time of the murder, and additional evidence taken from the Black Stump Motel earlier this year.

Sergt Darcy said each of 100 people contacted about the samples were happy to co-operate with police.

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DNA may solve 21-year-old murder case