A 37-year-old Melbourne woman has admitted impersonating police officers to extort more than $12,000 out of her vulnerable uncle.

The Victorian County Court heard Lydia Vonja, of Maidstone, preyed on her uncle because she knew he had money, was "a soft target" and a law-abiding person.

Her offending was described by judge Bill Stuart as "particularly cruel".

Vonja, a long-term heroin addict and user of the drug ice, pleaded guilty to one charge of blackmail and two bail breaches.

The court heard Vonja's 59-year-old uncle lived with his mother, Vonja's grandmother, suffered from paranoid schizophrenia and was "easily manipulated".

Vonja first rang his home in December last year pretending to be a constable from the Sunshine police station and demanded he put $200 in the letterbox for an unpaid fine, the court heard.

After collecting the first payment, the court heard Vonja rang her uncle on numerous occasions and demanded more money, threatening to cancel his licence if he did not pay.

She also pretended to be a detective from the Footscray police station.

On one occasion, the court heard Vonja accompanied her grandmother and uncle to a hospital appointment, and contacted her uncle later the same day to tell him he had parked incorrectly at the hospital and would need to pay another fine.

The offending later escalated to demands for thousands of dollars at a time, before the victim contacted Victoria Police, who told him they were not responsible for the phone calls and threats.

When he refused to pay the next time Vonja called, she threatened to kill him, the court heard.

Judge Stuart said the case presented "a very curious scenario" and was "unusual... to say the least".

"Once again, that dreadful drug ice comes to the fore," Judge Stuart said.

"Those who don't think ice is a scourge on our community need to think again."

Vonja 'not ready to leave prison'

Vonja's lawyer Rebekah Sleeth told the court her client was caught in a $250-a-day heroin and $50-a-day ice habit.

Ms Sleeth said Vonja funded her heroin addiction through lawful means for a number of years, before ice took over.

"When introduced to the drug ice two years ago her life spiralled out of control," Ms Sleeth said.

Ms Sleeth told Judge Stuart, even though Vonja was eligible to be sentenced to a community corrections order, she was not ready to leave prison because "she didn't feel ready to be back in the community", and did not want to be exposed to the temptation of ice.

"She's started to settle down ... the drugs [are] leaving her system and she has a fear of leaving [prison]," Ms Sleeth said.

Ms Sleeth said her client was "tremendously ashamed" of her actions.

"In her own words, her uncle is a law-abiding, good, simple, kind man," Ms Sleeth said.

"This wasn't sophisticated... she consents to a compensation order in his favour."

Judge Stuart said he was "deeply concerned" that Vonja did not want to leave prison.

He ordered her be assessed for a rehabilitation placement, ahead of sentencing.