Sergeant Tony Woodbridge, of Oamaru, said through police investigations and assistance from the public, a group near where the incident occurred at the rail crossing on Severn St, near the North Otago RSA building in Itchen St, had been identified.
He said police would speak to them in due course as part of their investigation, which was ongoing.
The incident took place near the Oamaru skate park, but it had yet to be established if those responsible were regular visitors to the recreational area.
On Friday, Acting Sergeant Carl Pedersen told the Otago Daily Times the incident was a serious one.
"The indication would be that some plonker has thrown a rock at him and hit him," he said.
"And ‘plonker’ is probably a very kind term to the person, because it’s obviously potentially very, very serious over and above the injuries the person has sustained."
St John treated the driver at the railway station. KiwiRail acting chief operations officer Henare Clarke said the driver’s injuries were minor, but he was relieved as a precaution.
