An electrical malfunction is to blame for the weir gates opening. The artificial lake is now less than half full and is likely to remain so until the next heavy rains.

The spill exposed tonnes of rubbish resting on the lakebed.

“If you were collecting shopping trolleys you’d really have scored,” said the officer in charge of police water operations, Senior Sgt Bob McDonald.

“We found motorbikes, park benches, and what we suspect is a stolen car.”

Police tried to inspect the car to check there was no body inside, but had to abandon the job because of warnings over the algae.

“We’ve been told that environment is not safe for us,” Sgt McDonald said. “We’ll have to wait.”

It’s the first time in years that the lake is near-empty.

The Adelaide City Council issued a release late yesterday warning people not to walk into the mud in the riverbed to retrieve submerged items, saying: “This may cause injury, or result in someone becoming stuck in the mud.”

It said the lake held around 500 megalitres, and the malfunction on the River Torrens weir saw the water levels drop by around two metres (around 400 megalitres).

It is estimated that around two to three millimetres of rain needs to fall into the catchment zone to refill the lake.

Lord Mayor Michael Harbison admitted yesterday that the malfunction was an embarrassment for the city.

"It's a very embarrassing thing and we certainly wish it hadn't happened," he said. "It's certainly not good, but what can you do?"

The South Australian opposition described the malfunction as a debacle and one made worse by the state government's inaction on securing the city's water supplies, AAP reported.

Opposition water security spokesman Mitch Williams said if the government had invested more in stormwater harvesting, there could be enough water stored in underground aquifers to refill the lake.

The council said there was enough water remaining to sustain birdlife, and staff were regularly checking the area for distressed animals.