CANBERRA -- The perpetrators of the terror attack at Sydney's Bondi Beach in December demonstrated high levels of awareness to hide their plans, Australia's intelligence chief said on Tuesday night.

Mike Burgess, director-general of security of the Australian Security Intelligence Organization (ASIO), told a Senate hearing that law enforcement and intelligence agencies did not know that the perpetrators of the Bondi attack were planning anything prior to the fatal mass shooting on December 14.

ALSO READ: Aussie PM apologizes to Jewish community for Bondi Beach terror attack

"It appears the alleged terrorists demonstrated a high level of security awareness to hide their plot. In simple terms, they went dark to stay off the radar," he said.

Fifteen people were killed in the attack, which targeted an event celebrating the Jewish festival of Hanukkah, making it Australia's most deadly mass shooting since 1996.

READ MORE: Australian PM announces terms for review into Bondi Beach attack

One of the alleged attackers, Sajid Akram, was shot dead by police at the scene while his son, 24-year-old Naveed, was arrested and charged with 59 offenses, including 15 counts of murder and one count of committing a terrorist act.

Naveed Akram was in 2019 investigated by ASIO over alleged links to a Sydney-based Islamic State terror cell, but was deemed not to pose a terror threat.

Burgess said on Tuesday night that he commissioned a review of the previous investigation immediately following December's attack, which concluded that the Akrams did not intend to engage in violent extremism in 2019.