Shayden Thorne, 25, has been held in a jail outside Riyadh for almost 18 months, reportedly after a laptop, which his family says he borrowed from a mosque, was allegedly found to have terrorist material on it.
A spokeswoman for Foreign Minister Bob Carr confirmed his detention and urged a speedy resolution to the case.
"We know that he has been charged with terrorism-related offences, but that is all we've been informed of at this stage," the spokeswoman told AFP.
"The minister requested then that the case be finalised in a timely manner and we're still hoping that this will happen," she said.
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The Australian Broadcasting Corporation said the man's 23-year-old brother, Junaid Muhammed Thorne, was also jailed for several months after protesting against his brother's arrest, before being released.
It reported that his passport has been confiscated by Saudi authorities and he was in hiding.
Junaid said Shayden, who is originally from Perth but has been living in Saudi Arabia for a decade, had been tortured in prison.
"They took a very, very long time to charge him," he added.
"I mean, he stayed for a year-and-a-half, a total of 18 months without any charges. And then suddenly out of the blue came, I think, six to seven terrorist charges, which is very, very weird, with no proof at all."
Their mother, who lives in Perth but did not want to be named, said Australia was not doing enough to get her sons home.
"If they've got evidence against them OK, fine, but there's nothing.
"I just think that they (Canberra) are not doing as much as what they should be doing to get my boys home.