Hydrogen peroxide will be added to Torrens Lake in the centre of Adelaide to treat blue green algae.

The South Australian Government has tried a number of methods to eradicate the algae, including flushing the lake several times with fresh water from upstream reservoirs over summer.

This has resulted in murky, nutrient-rich water pouring out of the Torrens Outlet at West Beach and into the Gulf of St Vincent, prompting warnings by officials to stay away from discoloured water at the popular beach.

The brown water has been spotted stretching 2.5 kilometres north to Henley Beach following the flushing.

State Environment Minister Ian Hunter said trials of hydrogen peroxide had been successful in smaller pools alongside the lake.

"If there's a question about putting too much peroxide in, then we will just withhold it and concentrate on the flushing instead," he said.

"But by having the two mechanisms, we can actually reduce the amount of water that we need to flush and use hydrogen peroxide to keep the levels of the blue green algae cells right down to a bare minimum."

Cam Walker from the Friends of the Earth environmental group said the use of chemicals had to be closely monitored to ensure native species were not affected.

Torrens Outlet at West Beach on March 2 after the waterway was flushed with fresh water. ( Supplied: Olga Marcus )

He said that while hydrogen peroxide was the safest of the chemical treatment options, it was also a known animal carcinogen.

"We know that animals like amphibians are particularly prone to being impacted because of the very porous nature of their skin and their ability to draw chemicals into their system very rapidly," Mr Walker said.

"So the key to this is to make sure the science is correct."

Mr Hunter said the dosage had to be exactly right to minimise the impact on aquatic life.

"But blue green algae seems to be incredibly susceptible to hydrogen peroxide and, at the right concentrations, the nature vegetation and animals don't really notice it," he said.

He said the chemical would be trialled in a small area of Torrens Lake.