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The joint investigation follows a report today in Sydney's Daily Telegraph that the 23-year-old contacted one player via Instagram before two players exposed themselves during a Facebook video chat.
In screenshots taken by the girl one of the players shows off his genitals while another bares his backside.
Several of the Rabbitohs biggest names were involved in the video chat but the Daily Telegraph chose not to name them or publish the screenshots.
The incident is alleged to have occurred while the Rabbitohs were celebrating their round 12 victory over the Warriors in Auckland on May 26.
The woman later emailed the club to complain about the incident revealing how a "funny" FaceTime chat "with the group of boys" turned explicit and left her "violated and disgusted", according to the Daily Telegraph.
The woman says phone records show one player repeatedly called her, even after she had asked to be left alone.
Ahead of their semi-final clash against St George Illawarra at ANZ Stadium tomorrow night, Rabbitohs coach Anthony Seibold denied knowing any details about the allegations and said he was focused on preparing the side for their sudden-death match.
"It's a little bit of chaos thrown in front of us, certainly not ideal," Seibold explained at a scheduled media session at Redfern Oval this morning.
"The NRL integrity unit, from my understanding, have known about this for a couple of days, I don't know how long they've known about it for, what the communications channels were and why it is the focus on the day before a semi-final.
"At the moment they're accusations, I have not seen any photos myself personally, I have heard the accusations, I am not willing to make any comment until the investigation is completed."
The Rabbitohs have confirmed the club's football operations manager Brock Schafer received the woman's initial email on June 1.
Schafer replied to the initial email assuring the woman the matter was being taken "seriously" and the club "will work to understand what has occurred". However subsequent emails from the woman on June 5 and August 31 were never responded after which she alerted the media.
"I feel quite disappointed that my email has not been responded too (sic) since a few months ago," she wrote in August. "I find it disgusting how these boys think they can get away with this crap."
A Rabbitohs spokesman said the club had hired an "independent contractor" to trawl through emails to verify if the woman had sent follow-up emails. He said this contractor could "confirm" that these emails were not answered because they "were tagged as spam".
"As per the system junk email policy these emails were subsequently routed to a junk mail folder," he said.
The Rabbitohs today issued a statement this morning saying the club will investigate the matter together with the NRL's Integrity Unit.
"The South Sydney Rabbitohs will investigate claims in today's media regarding alleged player behaviour in May this year," the statement explained.
"A thorough investigation will be undertaken in conjunction, and in close operation, with the NRL's Integrity Unit.
"Since receiving contact details last night for the female involved, Rabbitohs officials have attempted to make contact via phone and email.
The Rabbitohs take any allegations of player misbehavior very seriously and will not be making any further comment until the completion of the investigation."