A Melbourne psychiatrist who sexually assaulted a vulnerable long-term patient has been slammed for the "absolutely catastrophic" impact of his abuse of power.

Prabakar Rajan Thomas, 69, pleaded guilty to a charge of sexual assault today, more than three years after his attack on the woman who had gone to him for help.

"He must have understood the psychological devastation he would be wreaking," County Court Judge Liz Gaynor said in a pre-sentence hearing, slamming the doctor's breach of trust and labelling it as "absolutely catastrophic".

Prabakar Rajan Thomas, 69, pleaded guilty to a charge of sexual assault (Nine)

The woman attempted to take her life after the November 2015 assault.

Addressing the victim directly, the judge offered reassurance.

"You are not abnormal for feeling that way. It is one of the cruellest outcomes," she said.

"It is in no way your fault."

Judge Gaynor said it was heartbreaking for her to see time and time again victims blaming themselves, particularly this woman who was already dealing with deep-seated psychological struggles.

She was first referred to Thomas for treatment of bipolar in 2011 and began attending regular appointments.

Two years later, after discussing the effects of medication on the woman's sex drive, Thomas began asking unnecessary questions about her masturbation habits, prosecutor Sharn Coombes said.

He showed her graphic adult pornographic material, rubbed her backside and called her sexy when she complained of weight gain caused by medication.

Prabakar Rajan Thomas, 69, showed the victim graphic adult pornographic material, rubbed her backside and called her sexy (Nine)

Struggling on the anniversary of a stillborn child, she sought help, but Thomas again quizzed her about sex and masturbation and told her he'd had an affair with a patient previously.

She complained to staff at suburban Delmont Hospital, where Thomas had leased space since 2005, calling him "sleazy".

He assaulted her at the next appointment, which she made only to have a prescription filled.

Thomas put his hands in her bra and exposed her breasts and told the woman to masturbate for him, touching her over her clothes.

Ms Coombes said the woman sat frozen with her legs crossed during the attack.

The woman tried to take her life two days later, detailing the attacks in a note, having already reported Thomas to police and the medical board.

He voluntarily handed back his licence three years later, last November.

Defence barrister Nick Papas said Thomas was remorseful and regretted that the woman was cross-examined during an earlier court hearing.

Thomas will be sentenced later this month.

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