Police said the relationship between the victim and the offender had not yet been established.
A scene examination today uncovered a firearm believed to be involved in the incident.
Police said there was not thought to be any risk to the public.
Officers were wanting to hear from anyone who was in the Kelvin Street area at the time of the shooting, who might have relevant information.
Anyone with information was being urged to call the Southern District Command Centre on (03) 471 5002.
Invercargill resident Caitlin Barton said she saw a man lying on the footpath as she drove to work this morning.
"He was lying flat as if he had slipped on ice and had a back injury. Lots of people were crowding around him and an ambulance came flying past me ... I didn't think it was a shooting until I saw it on Facebook," Barton said.
Residents on Kelvin St posted online saying they could see the armed offenders squad from their homes, and didn't think they were allowed outside yet.
Bill Mackintosh, who lives on Kelvin St, said there were a lot of armed police in the area, including a dog van, but no one really knew what was going on.
"I went out to the supermarket and on my way home I came past a policeman with an assault rifle. He let me through to my house, though, and didn't say to stay inside," he said.
"My neighbour went down to find out what happened and thought the person might have taken off on foot, but I'm not sure if they've tracked them down yet or not."
