Casey Johnson, the troubled heiress to the Johnson & Johnson fortune, is under arrest for grand theft after she allegedly stole dozens of pieces of clothing, jewelry and other items from a supermodel who had befriended and tried to reform her.

Jasmine Lennard said she was away from her Hollywood home for six weeks and returned 12 days ago to find her apartment had been plundered.

She told The Post that the thief stole jewelry, shoes, 600 pages of a legal document, clothing -- even her underwear.

The thief also left a bizarre calling card -- a used vibrator was found in her bed and a wet towel was on the floor.

Lennard suspected Johnson, whom she said had "been like a little sister to me."

Then she got a text message from Johnson's off-again, on-again girlfriend Courtenay Semel. Lennard is a friend of both women.

"There's a problem, Jaz, Casey Johnson just got into bed with me and she is wearing your underwear," the message read. "You need to call police. There are documents here, your shoes and your clothing -- you need to call the police."

Johnson is the great-granddaughter of the founder of the Johnson & Johnson pharmaceutical empire and is the daughter of Woody Johnson, owner of the New York Jets. Her godmother is Diandra Douglas, the ex-wife of actor Michael Douglas.

The 30-year-old socialite has been linked to several high-profile girlfriends, including Semel, the daughter of Yahoo! CEO Terry Semel.

Semel allegedly beat Johnson up and lit her hair on fire during a catfight at Semel's home. But they apparently reconciled and were seen together earlier this year.

Lennard said she had never had sex with Johnson.

"Since the day I met Casey, I have only been a good force in her life. She's been like a little sister to me," she told The Post.

"I tried to get her off drugs and alcohol. I tried to get her into a 12-step program. I tried to help take care of her daughter. I've given her money," she said.

"I am the only person who helped this girl, and I believe she was obsessed with me, and thinks in her mind we had some kind of affair. That is not the case."

She said she was outraged: "She breaks into my Hollywood apartment, masturbates in my bed, has a shower in my bathroom, takes every thing in my apartment."

"This time she really messed with the wrong lady. I am going to teach her a lesson," Lennard vowed.