So many shopping trolleys have been dumped off a wharf in a regional South Australian city that the local council has warned of a "significant risk of injury" from jumping off it and into the sea.

Key points: Port Augusta City Council warns people not to jump off town wharf

Port Augusta City Council warns people not to jump off town wharf Divers found 500 trolleys beneath the water off the structure

Divers found 500 trolleys beneath the water off the structure Council and retailers to work to remove the trolleys

Divers employed by the Port Augusta City Council in conjunction with Coles, Woolworths and Big W have found about 500 submerged trolleys off the town wharf.

Mayor Sam Johnson said the trolleys belonged to supermarkets positioned near the water's edge and have been building up in the water for at least the past few years.

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He said people should take care around the area while the council and retailers figured out a clean-up plan.

"Being a recreational area and being the water, you'd naturally expect people to be swimming in that area and recreational fishing etc," Mr Johnson said.

"If you'd asked me to guess how many were down there, I would have said 50 initially … but the divers identified some 500 trolleys."

People found "fun and humour" throwing the trolleys in the water, Mr Johnson said.

The problem came to the fore in May when a low tide revealed just how many trolleys were in the Spencer Gulf.

The council then commissioned the survey, which came back with what Mr Johnson described as a "quite alarming" result.

Retailers expected to contribute to cost

Under South Australian law, areas below the high-tide mark belong to the State Government.

But since the trolleys belonged to the retailers, Mr Johnson said they were expected to pay for much of the cost of the removal, to be done by crane in coming months.

"Ideally, the best situation would be to make sure the retailers pay for it, but if there needs to be a contribution from other parties we'd like to see DPTI [the Infrastructure Department] or the State Government come to the table to make that contribution," he said.

"I don't think it's rightfully fair that ratepayers should be expected to foot the bill — and especially not 100 per cent — but we do know there are some safety concerns and by way of identifying that council is getting some signage erected to make people aware of some of the hazards that could arise beneath the wharf.

Trolleys underwater off the Port Augusta wharf. ( Supplied: Barry Kenneth Lewis )

"Also, just recently we upgraded our CCTV camera system throughout our CBD which includes that wharf area."

Port Augusta Masters Swimming Club president Anne Baker said members tried to avoid swimming near the wharf because of the risk of cuts from the trolleys, which are covered in barnacles.

"At low tide, you could touch them with your feet if you're upright in the water," Ms Baker said.

She said the group tried to remove two trolleys but found it too hard.

"It was just too big of a job and to access the wharf in that area would be hard," she said.

Port Augusta's last remaining jetty closed four years ago because it was structurally unsound.

The city is one of the hottest in South Australia and one of the farthest north location with sea access.