A homeless man and a woman have pleaded guilty to federal charges in the United States over a GoFundMe scheme that netted $US400,000 with a phony story about him coming to her aid.

Johnny Bobbitt conspired with Katelyn McClure and her then-boyfriend Mark D'Amico to make up a story in 2017 about Bobbitt giving McClure his last $20 for fuel when she was stranded on a Philadelphia highway, according to prosecutors.

Less than an hour after the couple set up the page to solicit donations, McClure sent a text message to a friend acknowledging the story was "completely made up", prosecutors said.

"I had to make something up to make people feel bad," Ms McClure said in a text — one of 60,000 reviewed by prosecutors — to a friend.

The scheme raised $US400,000 ($550,000), which the couple claimed would be donated to Bobbitt.

Instead, New Jersey authorities said, the three split the money and spent lavishly, including on a car, designer bags, and trips to Las Vegas, Disneyland, Disney World, the Grand Canyon and New York City.

Bobbitt, 36, of Philadelphia, pleaded guilty to conspiracy to commit money laundering, and McClure, 28, of Bordentown, New Jersey, pleaded guilty to a charge of conspiracy to commit wire fraud.

GoFundMe said it had refunded all the donations.

However, the fraud did not stop with the GoFundMe page.

The trio did interview after interview, posed for photos together, revisited the spot where they claimed their first encounter happened and went on Good Morning America.

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Their relationship soured, though, when Bobbitt sued the couple over what he said was their failure to turn money over to him.

New Jersey prosecutors said the suit led them to start an investigation, including hauling away a BMW they determined was purchased with the funds at the New Jersey home where McClure and D'Amico were living at the time.

"The entire campaign was predicated on a lie," Burlington County prosecutor Scott Coffina said late last year.

"It was fictitious and illegal and there are consequences."

Mr D'Amico's attorney, Mark Davis, said his client denied any wrongdoing and had not been charged by federal authorities.

But all three still face charges in state court.

Johnny Bobbitt pleaded guilty to conspiracy to commit money laundering. ( AP: The Philadelphia Inquirer )

ABC/AP