Saudi officials arrested Shayden Thorne, of Perth, 18 months ago and have detained him in a prison near Riyadh, the Australian Broadcasting Corp. reported.
The exact nature of his alleged crimes wasn't reported.
His brother, Junaid Thorne, 23, is also in Saudi Arabia and said officials confiscated his passport.
"I'm in hiding actually; any checkpoint or any police officer that stops me and asks for my identity card and realizes that I am Muhammed Junaid, I'd be arrested straight away," he told Australia's NITV.
Junaid Thorne said he was briefly detained for taking part in a protest.
The men's mother, who spoke on condition of anonymity, said she was worried about her sons.
"[Junaid] contacted me and said he feared for his safety," she said. "He thought they would arrest him again and he didn't want to go back to prison, he just wants to come home."
Consular officials have been giving assistance to Shayden Thorne since his November 2011 arrest, ABC said.