A Queensland couple unfamiliar with the layout of the central business district in the central western NSW town of Dubbo made the wrong decision when they attempted to elude police by running down the laneway leading to a former prison.

The Old Dubbo Gaol is surrounded by 3.6 metre high brick walls and has only one entry and exit point. With no place to go and few places to hide, the 23-year-old man and 31-year-old woman were quickly arrested.

The couple had earlier aroused the suspicions of staff at Priceline Pharmacy in Macquarie Street.

Police said the woman had presented a prescription for the opioid prescription painkiller Oxycodone during the busy lunch-time rush.

The drug, commonly known as hillbilly heroin, is sought after for unauthorised and criminal use throughout Australia and overseas.

It is highly restricted under Schedule 1 of the Misuse and Trafficking Act and Schedule 8 of the Poisons and Therapeutic Goods Act.

Without valid and legitimate prescription the possession, supply and use of Oxycodone is a criminal offence. Pharmacist Mark Rugendyke said the woman appeared pleasant and well-dressed.

The man, described as being "skinny with a dodgy haircut", waited outside the pharmacy before sitting on the steps of a nearby building. "As soon as I saw the prescription I knew something was wrong,'' Mr Rugendyke said.

"It was hand-written by a doctor in Victoria. Oxycodone tablets had been added to the bottom. The entire prescription had been traced over with another pen in a badly executed attempt to disguise what I considered to be a fairly obvious forgery.

"I rang the doctor to confirm the prescription and he said Oxycodone had not been ordered.

"It was a typical golden triangle of prescription forgery with the patient, doctor and pharmacy all in different locations.''

Mr Rugendyke contacted police and asked the woman to wait while the prescription was processed.

The pharmacist went into Macquarie Street to check the man's movements before returning to the dispensary.

"The man was keeping an eye on a car parked in a loading zone,'' Mr Rugendyke said.

"The woman got sick of waiting for the prescription and left the pharmacy to join the man in Macquarie Street.

"I kept an eye on them and pointed them out when police arrived.

"The man and woman ran into the laneway near Old Dubbo Gaol. The man was taken into custody near the entrance. The woman disappeared inside and was located after about five minutes.

"They were both arrested and the car was impounded. Police told me the car had stolen number plates and forged cheques were found inside.''

Police confirmed the man and woman were arrested. The woman was charged with forging a prescription for a prescribed restricted substance and having goods in her custody suspected of being stolen.

She was granted conditional bail to appear in Dubbo Local Court on January 6. Further details about the man were not available.

Mr Rugendyke described Oxycodone is a much abused drug.

"A lot of work has been done to crackdown on its supply,'' he said.

"The introduction of computer generated prescriptions has reduced the number of forged prescriptions being presented to pharmacies but offenders continue to try to obtain Oxycodone illegally.

"This latest attempt was a funny sort of incident. The prescription had clearly been altered and the people involved obviously didn't know anything about the layout of Macquarie Street or the Old Dubbo Gaol.''

Old Dubbo Gaol began as a courthouse lockup in 1847. It was proclaimed a gaol in 1859. A new gaol was built in 1871. It was closed in 1966 and reopened as a tourist attraction in 1974.