Wayne Patrick Clancy, 59, pleaded guilty on Tuesday to three counts of threatening to commit a sexual offence while in jail at the Hopkins Correctional Centre.

Prosecutor Jamie Singh told the Ballarat Magistrates Court that on January 27 2017 at 1.40pm, a letter was located by prison staff which was addressed to the female victim, who was a corrections worker.

The letter included a crudely drawn penis with an arrow towards a pornographic image of a female, next to expletive laden threats.

Mr Singh said the letter was written poorly with upper and lowercase letters, and named the woman repeatedly.

The letter was not shown to the victim and was placed in an evidence safe.

Another note with similar threatening sexual content was found in a multi-purpose room on February 2.

On February 6, a third letter addressed to the victim was slipped under the door of the multi-purpose room, with a hand-drawn penis pointing towards a drawing of a woman's backside and more threats of rape.

Mr Singh said the woman was "in shock" when she opened the letter, which had a fingerprint matched to Clancy's on it.

In a victim impact statement read to the court, the woman said she felt she could never return to Hopkins Correctional Centre, and had experienced a mental breakdown due to post traumatic stress disorder.

The court heard she was socially-isolated, had episodes of being "physically sick in the toilets" at work due to her anxiety and said the incident had a "permanent and ongoing effect on her employment". She continues to work four days a week at another facility.

Defence lawyer Tim Bourbon said Clancy had an "utterly tragic upbringing".

Mr Bourbon said a fall from a second story housing commission home caused the man brain damage at the age of two.

By the age of 12, Clancy was committed to the insane asylum J Ward in Ararat where he was sexually assaulted, despite not being convicted for any crimes.

Mr Bourbon said it was "possible to conjure up more significant examples" of the threatening to rape offence as it did not include weapons or touching.

He argued any sentence should run concurrently and not interfere with his non-parole period for a previous rape conviction, which will finish in April 2021.

But prosecutor Mr Singh said the charges were "serious examples of serious offences" where he was genuinely threatening the woman with harm and had a "real impact" on the victim.

He said the victim had attended more than 30 sessions with a psychologist and had to reduce the amount of prisoner contact she had in her role. Mr Singh said the mental health professional had noted the woman had been subjected to "rumours amongst staff" when information about the threats was withheld from her.

Clancy appeared in the dock with grey hair cropped at his shoulders, a goatee and beard and gold wire-rimmed glasses

Magistrate Gregory Robinson noted the man's history provided a "complicated set of circumstances", but said the incident had "ruined" the victim's career and capacity to work in her chosen field.

He said as Clancy had not pleaded guilty at the earliest opportunity, with the case referred from the County Court only after the man agreed to plead guilty, the sentence could not be mitigated due to his remorse.

The court heard the maximum sentence on each charge is two years, or five years served concurrently, and he may be placed on the sex offender register for life.

He will return to the Ballarat Magistrates Court for sentencing in June.