Darryl Matsuda and Jason Green of the Bay Area News Group reported details of the suit. The woman says Hayne assaulted her in December 2015. The Santa Clara County District Attorney's Office investigated the case and declined to press charges in October 2016.
"The victim was a young impressionable woman out with friends in a location and environment she thought she could trust only to find that Mr. Hayne used her level of intoxication and his physical stature to deprive her of the ability to make a personal decision about whether or not she wanted to have sex," Micha Star Liberty, an attorney for the woman, said. "As a result, she suffered greatly emotionally and physically."
The woman, referred to as Ms. V, says she was a virgin at the time of the attack. She says she had vaginal pain following the event, leading to her consulting a doctor in April 2016. A police investigation was opened after the doctor determined she was sexually assaulted, though she did not talk to prosecutors until May.
The district attorney's office said there was not sufficient evidence to press charges.
"Having represented sexual assault victims for over 17 years, what we know is that the process of mustering the strength and courage to come forward and speak out takes time," Liberty said.
Hayne played for the 49ers during the 2015 season before returning to rugby.