Public Service Association general secretary Stewart Little says prison officers across the state will strike from 6am on Friday for 24 hours.

A Corrective Services officer, known as Officer A, was initially charged with manslaughter in February 2021 over the 2019 shooting death of an inmate near Lismore Base Hospital.

The officer pleaded not guilty in February this year and was due to face trial in October.

A NSW Office of the DPP official said new evidence had emerged.

"All the available evidence in this matter, including newly subpoenaed material, has been reviewed in preparation for the upcoming trial, and the director has determined that the appropriate charge is one of murder," the official told AAP on Thursday.

Mr Little says members are confused about the upgraded charge, which came more than three years after the March 15, 2019, shooting.

A 43-year-old Wiradjuri man had been taken into custody earlier that day after being denied bail over assault charges, and appeared to have an epileptic seizure in the cells at Lismore Local Court.

He was taken by ambulance to hospital by two officers and was discharged about 7.30pm after being treated.

He was handcuffed and shackled as he was being escorted back to a prison van when he allegedly elbowed an officer and ran off.

It is alleged that as he fled, the man was shot in the back and died from a single gunshot wound.