Adelaide's drinking water could be under risk of contamination and lead to outbreaks of gastro, because of the State Government's plan to open up reservoirs to recreational activities, a report has warned.

Key points: Last year, the Liberal Party made an election promise to open several SA reservoirs for recreational use

Last year, the Liberal Party made an election promise to open several SA reservoirs for recreational use The Myponga Reservoir opened to walkers and runners in April

The Myponga Reservoir opened to walkers and runners in April A report says the Government may be pushing too quickly

An independent annual report finalised in April revealed SA Water could breach Australian Drinking Water Guidelines if metropolitan reservoirs were opened by the end of 2019, risking an outbreak of respiratory and gastrointestinal illnesses.

"Permitting public access to the metropolitan inner catchments and source water reservoirs should only occur after due consideration of the possible direct, and indirect, consequences on safe water, public health and costs, and taking a precautionary approach in the face of any uncertainties," the report by Water Futures — a water management consultancy firm — said.

"However, the approach adopted has been to require the opening of the water sources prior to any such consideration.

"Such undue haste and lack of due process is not compliant with a precautionary approach."

Water Minister David Speirs opened the Myponga Reservoir, south of Adelaide, for runners and walkers in April.

By the end of the year, the State Government plans to allow recreational fishing at the dam, followed by kayaking and paddleboarding later on.

The Liberal Party also promised to open the Happy Valley, South Para, Hope Valley, Barossa and Tod reservoirs for recreation.

Many reservoirs interstate are open for recreational use.

Runners at the opening of the Myponga Reservoir in April. ( ABC News )

Government 'reckless' in opening reservoir

Labor water spokeswoman Susan Close said the Government would have known about the report, which she said was given to SA Water and SA Health during the same month the Myponga Reservoir was opened to the public.

Dr Close said delivering safe drinking water was a key function of Government.

"[The Government] hasn't put money into extra staff, and yet has pushed ahead and opened up Myponga Reservoir and is promising to open others soon," Dr Close said.

"We have a Government that is being reckless about our drinking water supply in order simply to stubbornly insist on opening reservoirs for recreation."

She said the Government must close the Myponga Reservoir until it had invested in the necessary infrastructure.

"The Government needs to apologise to the people of South Australia for knowingly putting their drinking water at risk and they need to either decide not to open up the reservoir or they need to find the money for proper filtration or for staff to police people," she said.

Minister says he is being cautious

Mr Speirs said South Australia's drinking water was not at risk and the Government had all of the checks and balances in place needed at this stage.

He said SA Health was leading the process and had signed off that it would be safe.

Water Minister David Speirs talks at the opening of Myponga Reservoir. ( ABC News )

"We're being cautious and methodical about it, working with independent scientists and SA Health to ensure public health is foremost — we've always said that," he said.

"Susan Close accuses me of being reckless, I'm going very slowly at this, there is nothing reckless about my activities."

He said staff were monitoring the reservoir at Myponga, and infrastructure upgrades would be happening down the track.

"We've got a much more detailed water quality analysis model which has been developed using what has happened in Queensland, because we know that … this is positive, and we know we have the experts in SA and we know that this is a popular policy," he said.

The former Labor government opened the Warren Reservoir in the Barossa Valley, the Beetaloo Reservoir near Crystal Brook and the Aroona Dam near Leigh Creek to fishing last year.