Cassidy (40) had been a police officer for 14 years, the Alexandra District Court heard.
She admitted the charge, which involved her failing to record or disclose a statement made by Senior Constable Neil Ford at a motor vehicle crash scene on Earnscleugh Rd, Alexandra, on July 7, 2005.
Cassidy was in charge of the investigation into the collision between Ford's Holden Rodeo and a Honda driven by Shane Cribb, then aged 17.
Ford (56) was jailed for 28 months in September after being found guilty of perjury.
He had claimed Mr Cribb was to blame for the accident and the teenager was convicted for careless driving causing injury.
Cassidy was the first police officer to attend the scene of the accident and heard Ford say: "I was turning into the driveway, backed out [and] saw the car coming.
"I then 'gave it the gun' to try and get out of the way."
Four days later, she saw Ford's statement, which was quite different from what she had recorded in her notebook, Judge Kellar said.
Mr Cribb was convicted on the driving charge, but a rehearing was ordered after his supporters found independent witnesses who disputed what Ford had said.
Mr Cribb was acquitted of the charge.
Cassidy had stated she was "bullied" by her superior officer not to disclose Ford's initial comments, Judge Kellar said.
The aggravating feature of the case was that, as a police officer, she had committed a "breach of trust".
The information was material evidence and she failed to reveal it when Mr Cribb went to court on the driving charge.
It had taken four years before the defendant finally came forward with the information.
The mitigating factors included that she had been acting from "a sense of misguided loyalty" and that Cassidy seemed to have been "overborne by a commanding officer not to disclose the statement ", Judge Kellar said.
Police conduct that affected the administration of justice was always viewed seriously, he said.
Defence counsel Bill Dawkins, of Invercargill, said Cassidy's guilty plea was because it was her duty as a police officer to disclose what she was told at the scene "whatever pressures were placed on her".
There had been much attention in the community and media about the incident and he wanted to put Cassidy's actions into context.
Her version of events should be put before the court.
While it might not provide consolation for Mr Cribb and his supporters, it would give them an understanding of what happened and why she acted the way she did.
Ford and the defendant were work associates only, not friends, he said.
Cassidy said she had made Ford's comment known to a superior police officer and was told later to "go with the written statement" Ford made, even though it was different.
This comment had come under significant scrutiny and police said there was no evidence to support what Cassidy had said.
There were only two people in the office at the time the matter was discussed so it was a "he said, she said" situation, Mr Dawkins said.
Cassidy's confidence had started to diminish before the accident.
A psychologist she was seeing, as part of police policy, said she had talked in October 2005 of being bullied by a senior work colleague.
She was later diagnosed as being clinically depressed.
In 2006, she had a "breakdown" and was placed on medication. A senior officer had described her decline as the "Patearoa syndrome".
It was hard for her to maintain a tough facade in her vulnerable state and she was unable to stand up to pressure by her superior, Mr Dawkins said.
Cassidy had asked to be removed as head of the Ford crash investigation and had also asked for an independent crash investigator to review the matter.
When Ford was charged with perjury, Cassidy had confided in a police sergeant and disclosed the information she had.
Mr Dawkins said Cassidy did not consider it appropriate to ask people in the community for testimonials to present to the court.
The mother of a young offender had sent a testimonial to him and said, although she had no social connection with the defendant, she suspected Cassidy was "being made a scapegoat".
The district had lost a compassionate police officer, the woman said.
Thirty-four Alexandra Fire Brigade volunteers had also signed a testimonial.
Cassidy was embarrassed and "deeply ashamed" of the impact of her actions on Shane Cribb, her family and friends and the Alexandra community, Mr Dawkins said.
