Rugby league player Blake Ferguson has been convicted of indecently assaulting a young woman while out celebrating his State of Origin selection at a Cronulla nightclub.

Ferguson, who had claimed he touched only the woman's upper thigh in a case of mistaken identity, was found guilty of grabbing her vagina at the 2230 nightclub.

Ferguson's cousin, the boxer Anthony Mundine, and former Canberra Raiders teammate Josh Dugan were two witnesses at the hearing at Sutherland Local Court on Wednesday.

Mr Mundine provided a character reference, saying it was not in Ferguson's nature to treat women disrespectfully.

Mr Dugan described how he and Ferguson often had women throwing themselves at them when they went out.

The court heard that Ferguson and Mr Dugan started their night at the Northies hotel before moving on to 2230, where CCTV footage showed them drinking beers, hugging and talking to several women and posing for many photos.

Ferguson touched the vagina of a woman in a split-second encounter, before she then pushed him away.

He apologised in court and said he mistook the woman, who cannot be named for legal reasons, for another blonde he had spent time with earlier that night at Northies.

''I always liked blonde girls because I lived in Cronulla for a long time,'' he said.

''I would like to say sorry to [the victim], I would never touch her in a sexual way. I'm really sorry I mistaked [sic] her for the girl I hooked up with at Northies. I would never touch a girl like that if I didn't know them at an intimate level.''

Magistrate Jacqueline Trad rejected his story, saying there was no evidence to support it and, in any case, he showed no regard for the woman's consent.

He took ''no steps whatsoever to consider whether consent was being given and, in that sense, he was reckless'', she said.

Prosecutor Rick Mansley argued the ''mistaken identity'' excuse was a ''recent invention'' made up in court without a ''scintilla'' of evidence.

Ferguson's counsel, Adam Houda, asked for the sentencing to be adjourned to allow for a medical health assessment. He said Ferguson had raised this issue just moments before the judgment.

The court heard Ferguson who had struggled to deal with fame, had experienced a troubled upbringing. He attended six schools and was raised by his step-grandparents because both his parents were drug addicts.

''It was quite difficult getting, basically, girls thrown at me,'' he said. ''I still haven't … come to terms with all the attention with, like, photos and signatures and all that.''

Mr Dugan said he and Ferguson were often mobbed by female fans.

[''There is] no shortage of girls making advances at you?'' Mr Houda asked Mr Dugan.

''Not at all,'' he replied.

Magistrate Trad rejected Mr Houda's suggestion the victim of the assault was taking advantage of Ferguson's fame or had an unreliable memory of the incident because she was ''highly intoxicated'' from drinking three vodkas, one cider and two shots while at the Northies hotel.

Ferguson was dropped from the Origin squad and the Raiders because of off-field issues. He has not been signed by another club.