SOUTH Australian police have rejected claims by Senator Mary Jo Fisher that they exacerbated her mental illness by prosecuting her for shoplifting from a Frewville supermarket.

In a statement announcing her resignation last Senator Fisher accused SA Police of deliberately prolonging her prosecution for allegedly stealing groceries from the Frewville Foodland in December 2010.

The charge was later dismissed and Senator Fisher was found guilty of assaulting a female security officer who tried to prevent her from leaving the supermarket's carpark.

In her statement Senator Fisher said her attempt to recover from her mental illness - which included severe panic attacks - had been made more difficult because of the criminal charges brought against her.

"My faltering recovery was exacerbated by repeated pressures from dogged, calculated and deliberate attempts by the police prosecution service to do everything possible to increase the length, complexity, cost and emotional turmoil both of my recent and extended trial, and the subsequent and continuing post-trial cost processes."

However, SA Police Assistant Commissioner Tony Harrison said today that police had always acted in the public interest during her prosecution.

"Any assertion that there was a deviation from following appropriate and standard process when adjudicating or prosecuting this case is strongly refuted,'' he said.

"Considerations to determine whether to continue with a prosecution include the likely prospect of a conviction and whether it is in the public interest."

"SAPOL followed standard process in the management of this case. The Senator was found guilty of assault and not guilty of theft."

Asst Comm Harrison said Senator Fisher was now pursuing the matter of costs against SAPOL in the courts.

"As this process is ongoing it is not appropriate for SAPOL to make any further comment," he said.

11AM UPDATE:Liberal Senator Mary Jo Fisher has done the "gutsy thing" by quitting parliament after being accused of shoplifting a second time, Opposition Leader Tony Abbott says. t 10.

Mr Abbott says it was a tragic personal situation for the senator, who has done the honourable and the courageous thing by herself, her family and the parliament by resigning.

"I think the best thing we can do now is wish Mary Jo all the best as she puts her life back together again," Mr Abbott told the Nine Network.

Senator Fisher, 49, who had a very public shoplifting and assault trial this year, announced on Thursday she would resign after another panic attack.

The senator says the latest attack was similar to the one which caused "such grief" 18 months ago when she was first charged with stealing almost $93 of food from a supermarket and assaulting a guard.

She was found guilty of the assault but no conviction was recorded. She has been reported for shoplifting again and could face charges by summons.

The manager of opposition business at parliament, Christopher Pyne, says the Liberal party is "shocked and saddened" by the senator's resignation.

But he says they will move very quickly to find a replacement.

Nationals Senate leader Barnaby Joyce agreed the best place for the Senator to receive help was outside parliament.

"It's not something about a person being a thief, but possibly a person having some other problems that need to be addressed," he told reporters in Canberra.

Liberal Senate leader Eric Abetz said he was aware Senator Fisher had been battling with private issues for a long time.

"I am as devastated as she is that circumstances have led her to the conclusion that it is best for her to leave parliament.. ," he said.

Senator Fisher could leave with "great pride" having served South Australians for five years, he said.

"She was a very passionate, sincere and capable advocate for the people of rural and regional Australia," Senator Abetz said.

Labor backbencher Doug Cameron said: "I wish her the best for the future and hope that (she receives appropriate treatment for) this terrible mental illness that afflicts many people."

10:17am UPDATE: Prime Minister Julia Gillard says Mary Jo Fisher's decision to quit parliament must have been difficult.

The Liberal senator announced yesterday that she would resign on August 10 following a second shoplifting allegation.

Senator Fisher said she was resigning on medical advice after experiencing a further panic attack, similar but less severe than the one which caused such grief 18 months ago

"I'm sure that was a difficult decision for her in what was an unhappy set of circumstances," Ms Gillard said in Rio de Janeiro where she is attending a UN environment conference.

"She has taken the decision, no doubt a hard one, to return to private life."

Ms Gillard declined to comment further.

9:50AM UPDATE

THE Liberal party will move quickly to fill the vacancy of Senator Mary Jo Fisher, federal manager of Opposition business Christopher Pyne says.

He says the party is "shocked and saddened" to hear the Senator's mental health issues have forced her to resign.

The Senator, who faced court for shoplifting last year, announced yesterday, after being reported for shoplifting again, that she would resign from the Senate on August 10.

"It is terribly sad news," Mr Pyne told ABC radio.

"And the Liberal party and I are shocked and saddened.

"We all hoped that she would fully recover."

Senator Fisher said she was resigning on medical advice after experiencing a further panic attack, similar but less severe than the one which caused such grief 18 months ago.

She was then charged with shoplifting and assaulting a guard after taking almost $93 worth of food from a supermarket.

At the time she was found not guilty on the shoplifting charge but guilty of assault. However, no conviction was recorded because of her panic attack.

The Senator's statement of resignation did not mention she had been reported again.

Mr Pyne said being a senator was a highly stressful job and Senator Fisher also had to fight a difficult prosecution.

He said he hoped she would make a full recovery.

In the meantime, the party would "move very quickly" to find a replacement when parliament resumes in August after a winter break, Mr Pyne said.

Liberal MP Jamie Briggs said there were questions to be asked "about who made the decision to overturn the arrangement that the police came to with her legal team early on".

Last month, Senator Fisher's defence counsel, Michael Abbott, QC, told the Adelaide Magistrates Court she had proposed pleading guilty to an assault charge, with no conviction recorded, in return for the shoplifting charge being dropped.

Mr Abbott said the deal was backed by the two prosecutors in the case who recommended it be accepted.

"(But) somewhere, somehow, in the chain of command, the police were not willing to deal with the matter by any way other than a full trial," he told the court.

EARLIER

LIBERAL senator Mary Jo Fisher - who faced shoplifting charges - has quit federal politics amid new allegations of stealing from another suburban supermarket.

It follows a second alleged shoplifting incident at an Adelaide supermarket which she claimed had resulted from a panic attack.

The new incident took place at Coles supermarket at West Lakes on Saturday afternoon and involved the alleged theft of goods valued at $60.40.

The five-year senator announced her intention to resign in a statement yesterday issued about an hour after being contacted by The Advertiser to respond to questions on the matter.

She did not refer to the shoplifting claim, for which she has been reported by police, but revealed an ongoing struggle with "a depressive illness".

"I am today announcing my intention to resign my position as a Senator representing the people of SA, effective August 10, 2012," her statement said.

"I do so on medical advice and after experiencing a further panic attack, similar but less severe than the one which caused such grief 18 months ago.

"The road to full recovery from my depressive illness has been hampered by my trying to get better, at the same time as continuing to serve in public office."

She will leave the Senate just before it resumes sittings after the winter recess.

Her shock announcement came at 5.33pm, 68 minutes after The Advertiser contacted her.

Police confirmed "that a 49-year-old woman has been reported for shoplifting from a supermarket at West Lakes following an incident around 5.30pm on Saturday, June 16".

Police said security staff had "found goods in the woman's personal bags that had not been paid for."

In her statement, Senator Fisher - who was cleared of shoplifting but found guilty of a minor assault following an incident at a Glenunga shop last November - said she had since been persecuted by SA Police.

Her departure will cause a vacancy in the Senate to be filled by nomination of the Liberal Party and the State Parliament.

It will not affect numbers in the Lower House where the government is formed.

If you need help for depression phone Lifeline on 13 11 14 or Beyond Blue 1300 22 4636.