(AAP Image/Ed Jackson)
Grant Hackett's worried father is pleading with the troubled Olympian to make contact with his family, saying his son is "mentally disturbed" and needs help.
The triple Olympian hasn't been seen since Thursday morning and his father Neville is reporting the 36-year-old to police as a missing person.
Friends and family are out on the Gold Coast looking for him and Mr Hackett said it was "vital we find him as quickly as possible".
"He's definitely a missing person and he's mentally disturbed and needs urgent help," he told reporters.
"If anybody's seen him, contact the media or police or the Hackett family.
"Grant, let us know where you are, we love you and we want to help you."
Hackett was last seen at the Versace Hotel at 7.30am on Thursday.
Mr Hackett said his son was "very depressed and not in a good condition" when he was last seen.
He said the best-case scenario was he was currently in a hotel on the Gold Coast.
Hackett's disappearance is the latest twist in a deepening saga that started on Wednesday at midday when police were called to his father's house.
An agitated and verbally aggressive Hackett was calmed by police before being taken to the Southport watch house.
He was released without charge three hours later during which his brother Craig told media about the family's struggles with Hackett's "chronic" mental health issues.
Hackett hit back on Thursday morning, posting a picture on Instagram of himself sporting a black eye and cuts to his face.
Hackett accused Craig of assaulting him in what is believed to have been a separate incident last month.
"My brother comments to the media ... but does anyone know he beat the s*** out of me?" he wrote on Instagram.
"Everyone knows he is an angry man."
Australian Olympic Committee president John Coates issued a short statement saying Hackett would receive "any support he needs" from the committee.
"This is not the Grant we know and respect," Mr Coates said.
"Grant is a great Olympic champion, one of the greatest swimmers of all time, and his contribution to the Olympic movement and to sport in general should not be forgotten."
Fellow Olympian Daniel Kowalski says there had been warning signs about Hackett's mental health issues.
The Australian Swimmers' Association boss said he was hopeful Hackett would receive the help he needed to overcome his problems.
"You rely on people talking and seeking help and advice and going to someone, but it's a really taboo subject still," Kowalski told SEN Radio.
"(He needs) some tough love and (to) go to a place that won't let him check out of rehab early.
"Hopefully he can look in the mirror now and say, 'right, its now or never'."
© AAP 2017