A worker was reportedly forced to jump into the water when a fire broke out on the Whyalla jetty on Wednesday afternoon.
The Whyalla News understands contractors were performing angle grinding at the site when 75 metres of plastic decking caught alight.
Three MFS appliances supported by a CFS appliance contained the fire.
The man who was forced to escape the fire by diving into the ocean was treated by the State Emergency Service at the scene. Another worker was also present when the fire ignited.
Local Scott Barber said he knew something wasn't right when he noticed puffs of smoke above the jetty.
"In about five minutes the whole structure was up, it was a bit of a surprise. Once it started picking up we knew it wasn't being controlled," he said.
"One man jumped off the end and swam back to the shore...he may have had a bit of smoke inhalation."
Another local, Peter Barber, was manning the stall for his business Dessert Ice Creams when the fire began.
Whyalla Jetty caught alight around lunchtime after sparks from tools came into contact with combustibles. One worker jumped into the ocean to escape. Fire crews are on the scene. Video: Nathan Straya Myers, Facebook.@SkyNewsAustpic.twitter.com/HhDcSgxNDf— Emily Burley (@emilyburley) September 11, 2019
"Fortunately 99 per cent of what's out there will end up in scrap. It's a jetty that's under demolition orders so apart from a bit of air pollution it's been a very lucky incident," he said.
"The accident and incident investigation I'm sure they will have will probably find that there should have been better fire containment gear out there before the job was started.
"But hindsight is a wonderful thing it's very hard to be critical of a person because of something that's happened afterwards. I think they handled the fire as well as they could."
The damage bill for the fire is still being assessed, according to a MFS spokesperson.
The current jetty is set to be demolished to make way for the Whyalla City Council's new 'loop' jetty. Work began earlier this month with the jetty area and southern breakwaters being declared a construction zone. Contractors have been undertaking a soft strip of the jetty to remove railings and lights.
Whyalla City Council chief executive officer Chris Cowley said while the incident was regrettable, it was contained within the construction site now under control of Maritime Constructions, the company contracted to demolish the old structure and construct the new jetty.
"This appears to be an unfortunate workplace incident that has resulted in a fairly significant fire," Mr Cowley said.
"Importantly, we have had no reports of anyone being injured in this incident and it seems there was no danger, from the fire or smoke, to Whyalla residents.
"We will seek a debrief from Maritime Constructions on the reasons for this fire and seek further assurances that all safety measures are being observed in the demolition and construction to ensure there are no more incidents of this nature.
"We'd like to thank the fire crews and police that acted so quickly to put this fire out and ensure the safety of everyone in Whyalla."