A man and a baby girl have died after falling from the wall of an old dam near South Australia's Barossa Valley.

South Australia police have confirmed the deaths at the Whispering Wall in Williamstown, 50 kilometres north-east of Adelaide.

Emergency services were called at 4.30pm with a MedStar medical helicopter was also sent to the scene.

South Australia Police said they were investigating what happened.

"Police and emergency services were called to the Whispering Wall after witnesses reported seeing a man and infant go over the edge of the wall and fall to the ground," they said in a statement.

"When patrols arrived, they found a deceased man and an infant who was unresponsive.

"Paramedics worked on the infant but sadly, she died at the scene.

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"Detectives from Barossa CIB and Major Crime Branch, along with forensic crime scene investigators, have attended the scene to examine the circumstances surrounding the incident.

"At this stage, no one else is being sought over the deaths.

"Police will be preparing a report for the State Coroner."

The Whispering Wall is 35 metres high and is the retaining wall of the Barossa Reservoir, according to its website.

"Built between 1899 and 1903, the dam was a revolutionary engineering feat for its day and attracted attention from all over the world, even making its way into the pages of the journal Scientific American," it says.

"But what draws visitors to the Whispering Wall is its unique acoustic effects: words whispered at one side can be clearly heard at the other, more than 100 metres away."