A sickening one-punch attack outside a South Australian pub has left a 26-year-old man in hospital.

Footage obtained by 9News appears to show a heated confrontation between two groups before the situation suddenly turned violent last night.

The moment of impact outside Port Lincoln’s Pier Hotel is too graphic to show, as man falls to the ground after being knocked unconscious.

The pub was packed with locals and holidaymakers, with many quickly trying to help the man laid out on the footpath.

An ambulance was called to the scene on Tasman Terrace and the 26-year-old was taken to hospital just before 9pm.

Premier Peter Malinauskas said such attacks couild be deadly.

“We’ve seen tragically in the past where one punch, one erratic decision can cost someone their life and put someone else in jail,” he said.

“People are allowed to loosen up and have a few drinks but they’re not allowed to consume alcohol and then make completely irrational decisions that put other people’s safety in jeopardy.”

Police are still looking for the suspected attacker.

He is described as having a heavy muscular build and short dark hair and was wearing a black singlet and shorts.

The suspect was seen leaving with two men in a silver Toyota LandCruiser.

In that incident four men were taken to hospital after a brawl broke out between a group of men and security guards from the Colley Hotel.