The country's spy boss has detailed the extraordinary chance encounter between one of his young female employees and a group of accused jihadis, which allowed police to stop one of Australia's largest planned terror attacks.

Key points: A terror cell was planning an attack on Melbourne's CBD around Christmas 2016

A terror cell was planning an attack on Melbourne's CBD around Christmas 2016 They had bought precursors for explosives from a chemist, and were spotted acting strangely by a rookie officer

They had bought precursors for explosives from a chemist, and were spotted acting strangely by a rookie officer The chance encounter proved to be the big lead the spy agency needed to foil the attack

Details of how the rookie ASIO officer helped to foil the terror cell can be revealed for the first time after a court lifted a suppression order.

The jury took just over six days to deliver their verdict on November 2, after a lengthy trial that heard the men had purchased machetes as well as metal pipes, light globes and batteries to make explosives, and had tried to obtain guns.

ASIO director general Duncan Lewis has detailed how a woman who had only just joined the domestic spy agency, spotted a group of young Melbourne men "performing in an unusual way" in the days leading up to Christmas 2016.

"The Christmas crowds were gathering and we, as the result of the exertions of a young ASIO officer, a young woman who'd been with us for about five minutes, one of our analysts, had noticed a group of young men performing in an unusual way and going to a chemist's shop very late at night," he said.

"They were in fact picking up the precursors for explosives. That was all the lead we needed."

Ahmed Mohamed, Abdullah Chaarani and Hamza Abbas have been found guilty of plotting a terrorist attack. ( Supplied: Jeff Hayes )

His comments were made in a September address to military officials at the Land Forces conference in Adelaide, but for legal reasons they can only be publicly reported now.

During the men's trial, the prosecution detailed the steps the men took to plan the attack.

In early October 2016, both Mohamed and Chaarani conducted internet research on how to make improvised explosive devices (IEDs), including accessing an article about bombmaking in an Al Qaeda publication.

They made two trips to Bunnings in Broadmeadows, in Melbourne's north, in November and December to purchase supplies.