An Australian model has been locked up in a US immigration detention centre for two months after she was targeted in a "surprise ambush".

Adau Mornyang was fortunate to avoid jail in September when a US judge ruled benevolently in her favour, following the 25-year-old's arrest for slapping a flight attendant and going on an obscene tirade during a flight from Melbourne to Los Angeles.

Now Adau's sister, Maria Mornyang, has claimed the model has been illegally detained by US Immigration and Customs Enforcement (ICE) officials since September 17.

Australian-South Sudanese model Adau Mornyang arrives at the Federal Courthouse in downtown Los Angeles in July. The model and former Miss World Australia finalist Mornyang was in federal court for attacking a flight attendant and yelling a racial slur at an air marshall during a Melbourne to Los Angeles flight. (AAP)

On a fundraising page, Ms Mornyang alleges that ICE agents, upset her sister had narrowly avoided jail, exacted revenge by cancelling her US visa and throwing her inside a detention centre.

A Department of Foreign Affairs and Trade spokesperson told nine.com.au in a statement they were "providing consular assistance to an Australian detained in the US."

They could not comment further because of privacy obligations.

Mornyang's problems with US law enforcement and immigration officials began in January. She was nine hours into a January 21 flight from Melbourne to Los Angeles and had apparently been drinking wine excessively when she began "yelling obscenities and racial slurs and flailing her arms," prosecutors said during a September trial.

Her fellow passengers complained to the crew. But she only lashed out more when flight attendants attempted to calm her, and she slapped one of them. Several air marshalls had to come out from undercover to help deal with her.

Prosecutors had sought a month in jail for Mornyang, but US District Judge Cormac J. Carney sentenced her to three years of probation and 100 hours of community service.

Judge Carney said he believed Mornyang was truly sorry after she tearfully read a statement in court saying she is now receiving treatment for anxiety and depression brought on by childhood trauma, instead of self-medicating like she did at the time of the flight.

Prosecutors claimed Mornyang's statement of remorse said little about the passengers and crew whose flight she made so difficult.

Mornyang, a native of South Sudan who migrated to Australia as a refugee at age 10, was convicted of felony interference with a flight crew and misdemeanour assault. She was acquitted of a third count of assaulting an air marshall.

A copy of what is purported to be Adau Mornyang's US visa was posted on a fundraising page set up by the Australian model's sister. (Supplied)

On the fundraising page, Maria Mornyang posted an image purporting to show her sister's US visa, with an expiry date of December 2021.

"ICE agents had revoked her visa without notice because of an argument she had with a United Airlines flight attendant on January 21, 2019," she wrote.

"This no-jail outcome must've infuriated ICE officials who proceeded (without Adau's knowledge) to revoke her valid visa and subsequently arrested her for being in the country illegally."