Forensics officers began a search of the Mobil on Hillside Rd on Sunday morning and the area remained cordoned off.
The Otago Daily Times understands the assault took place late on Saturday night and involved two men, but did not involve any Mobil staff and the incident was not a robbery.
A police spokeswoman confirmed on Sunday they were investigating a ''a reported assault alleged to have happened on the forecourt of Mobil Forbury on Hillside Road around 10.30pm last night.''
A 43-year-old man was taken to Dunedin Hospital, she confirmed. A spokesman for the hospital said he remained in a serious condition in the intensive care unit.
A police spokeswoman said she understood no arrests had been made in connection with the incident.
The ODT understands a security firm has been contracted to provide an overnight guard at the station and an officer from the Criminal Investigation branch was at the Mobil on Sunday afternoon.
It was unknown when the Mobil would reopen.
Anyone with information in relation to the incident was asked to contact Dunedin Police on (03) 471-4800 or via Crimestoppers on 0800 555 111.
