Sulayman Khalid was handed the sentence at the New South Wales Supreme Court after pleading guilty to conspiring to act in preparation for a terrorist attack.
He was arrested during a series of police raids in Sydney and Brisbane in late 2014.
Four others, including an unnamed teenager, were jailed for between nine and 18 years for similar or lesser offences -- such as intentionally making a document connected with the preparation of a terrorist act.
Khalid "occupied a co-ordinating role" in the plot, Justice Geoffrey Bellew wrote in his ruling.
"In offending as he did, Khalid demonstrated that he was a devout terrorist," he added.
Canberra has become increasingly worried about homegrown extremism and officials say they have prevented 13 terror attacks in recent years.
But several have taken place, including the murder of a Sydney police employee in 2015 by a 15-year-old boy, who was then killed in an exchange of gunfire with officers.
In his ruling, Bellew said Khalid adhered to the "religious ideology of Wahhabi-Salafism".
"The cause that was to be advanced in the proposed terrorist act or acts was that of violent jihad... The offender had publicly displayed his support for the violent jihad espoused by ISIS (Islamic State)," he said.
The group had been planning to kill police officers and attack government buildings, and had considered using weapons such as firearms, Bellew added.
Australian counterterrorism police have made a series of arrests since late 2014, with the young age and radicalisation of many of those detained a growing concern for authorities.
Eyewitness? Submit your stories now via social or:
Email: eyewitness@pulse.ng
![The woman was recorded, among others, while bathing [BI]](data:image/svg+xml;charset=utf8,%3Csvg%20xmlns%3D'http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg'%20width%3D'300'%20height%3D'172'%20data-ring-placeholder%3D'1'%3E%3C%2Fsvg%3E "The woman was recorded, among others, while bathing [BI]")
