A "desperate" thief has been rugby-tackled in front of hundreds of shoppers after using a sledgehammer to steal gold necklaces from a jewellery store in Canberra's south.

Security footage showed the offender smashing a glass cabinet at the front of the shop and grabbing at least two pieces of jewellery about lunchtime on Tuesday in the Tuggeranong Hyperdome.

"It appears two bystanders have seen this taking place and one person has turned around — he's a large male — and attempted to hold the offender," Station Sargent Chris Meagher said.

"Unfortunately the offender got away from his grip and a short time later [another man] has also tried to tackle [the offender] in what could be best described as a reasonably good rugby tackle."

Station Sargent Chris Meagher said the nearby food court was particularly busy because it was school holidays and up to 250 people could have witnessed the robbery and scuffle.

"It's a brazen act. It does involve violence. Obviously that person is quite desperate," he said.

The offender, who was wearing gloves and hid his face behind sunglasses and a hoodie, is still at large.

"Aggravated robbery in the ACT carries 25 years imprisonment. It's a very serious offence," Station Sargent Chris Meagher said.

Police said they also wanted to speak to the two bystanders.

"We applaud them in trying to do it but encourage people not to take the law into their own hands," Station Sargent Chris Meagher said.

The officer declined to speculate on how much the jewellery was worth but confirmed it was returned to the store.