Australia's foreign minister summoned Israel's ambassador, saying it was a matter of "the gravest concern."
Australia seeks answers
The latest developments in Prisoner X's case prompted Australian Foreign Minister Bob Carr to request an internal report.
"I'm advised in the form of an interim report that the Australian government was informed in February 2010, though intelligence channels, that the Israeli authorities had detained a dual Australian-Israeli citizen, and they provided the name of the citizen, in relation to serious offenses under Israeli national security legislation," Carr told a Senate committee Thursday.
He did not mention what the alleged "serious offenses" were.
Carr said Australia sought specific assurances from Israel, such as that the detainee would get legal representation of his choosing and that he would not be mistreated.
"At no stage during his detention did the Australian government receive any requests from the individual or his family to extend consular support," Carr said.
"The Australian government was advised through intelligence channels on December 16, 2010, (of) this individual's death on the previous day, and the deceased's family had been notified by Israeli authorities," Carr said.
The Australian Embassy in Tel Aviv assisted in returning the body to Australia, Carr said.
Sharp criticism over secrecy
As authorities stay tight-lipped about details of the man's case, speculation continues to swirl.
The Association for Civil Rights in Israel said it sent a letter to Israel's attorney general about the case on Wednesday, criticizing the censorship and calling for the gag order to be scaled back further.
"What is far more concerning, of course, is the fact that a man was held in detention under heavy secrecy, and nothing was published about the reason for his arrest or the circumstances surrounding his death," wrote Dan Yakir, the association's chief legal counsel.
The letter argues that there is "considerable public interest" in more information about the investigation into the death of Prisoner X.
"Was it really suicide? Was there negligence in the supervision of the detainee? Has any official body taken responsibility? What steps have been taken to prevent the recurrence of similar events in the future?" Yakir asked.
Israeli government officials have made no further comments on the case.