South Australian Liberal Senator Mary Jo Fisher admitted Friday in her trial in Adelaide that she unintentionally took groceries as part of her depression which caused her to lose control and plunge into depths of despair.

The 48-year-old senator, who told the Adelaide Magistrates Court that her problem started in early 2008, pleaded not guilty to shoplifting $92.92 worth of food from the Frewville Foodland outlet in Adelaide on Dec. 15, 2010. She also filed a not guilty plea to assaulting a security guard who tried to prevent her from leaving.

Fisher, who was diagnosed with chronic depression in October 2009, recalled that she was feeling out of control and anxious on that day when she stopped to buy items on her way home.

Store security officer Cathryn Groot noticed that Fisher was leaving without paying for some items the senator had on her trolley. Groot approached and questioned her, which caused the senator to feel bewildered.

Fisher said she wanted to co-operate with Groot in paying for the items, but became confused when she was informed that the police would be the ones to handle the problem.

"I didn't understand how that was consistent with she and I sorting this out amicably and I didn't see how that was consistent with me paying for the goods I had apparently left the store without paying for," news.com.au quoted Ms Fisher.

Groot said she noticed the senator place some of the items from her trolley on the floor of the supermarket, returned some to the trolley and held some on her hands. The security officer pointed out that the way the groceries in the trolley were packed was suggestive that Fisher was about to do something dishonest, Ms Groot explained.

The incident led Fisher to seek medical assistance, and she was prescribed medication. Her defence counsel, Michael Abbott, said during the cross examination that he also disclosed to the arresting officer that Fisher had seen a psychiatrist at least four times prior to the shoplifting incident.

After the Frewville Foodland incident, the senator recounted she had a second experience of being out of control when she presided over a media conference as chair of the Senate committee that investigated the government's home insulation program.

"It was as if I wasn't doing the press conference, as if someone else was doing it and I wasn't even sure what that someone else was doing," Fisher recalled.

Fisher was first elected senator in 2007 after Howard government frontbencher Amanda Vanstone resigned. In March she also created controversy when she danced in the upper house as her way of mocking the government's policy of climate change.

The trial will continue on Saturday. Abbot's lawyer is expected to submit on that day a petition to for the court to declare Groot's evidence not admissible.