An Australian court sentenced a Sydney teenager to at least 12 years in prison on Tuesday for planning an attack inspired by the Islamic State group for which he bought knives and bayonets from a gun shop two years ago.

The now 18-year-old male, who cannot be named for legal reasons, was arrested outside a Muslim prayer hall in the western Sydney suburb of Bankstown in October 2016, and found guilty of plotting an attack by a jury in September.

Handing down his sentence at the Parramatta Supreme Court in western Sydney, Judge Geoffrey Bellew rejected the man's claim that he had purchased the weapons for hunting animals.

"At the time of his arrest, the offender was ready, willing and able to carry out a terrorist attack," the judge said.

"It reflects a deep and unstinting motivation to act upon and put into specific effect the irrational, immoral and heinous advice propounded in extremist propaganda issued by Islamic State," the judge added.

The teenager acknowledged to the court that he sympathized with the Islamic State group when he bought the weapons, but denied any plan to use them in a terror attack, saying he wanted them for camping and hunting.

Bellew said there was overwhelming evidence against the teenager, and sentenced him to 16 years in prison with a non-parole period of 12 years.

The judge said it was significant that the teenager's arrest came little more than a month after he had downloaded an IS magazine which mentioned Bankstown in an article urging readers to "alleviate the pain afflicting the hearts of the Muslims by striking the kuffar in their homelands."

Kuffar is a derogatory Arabic word meaning nonbeliever.

"Kill them on the streets of Brunswick, Broadmeadows, Bankstown, and Bondi," the magazine said, in reference to suburbs of Melbourne and Sydney. It also mentioned the Melbourne Cricket Ground and Sydney Opera House.

The judge said the teenager "regarded the government as evil," and said he was satisfied an attack was imminent when the offender was arrested with an alleged co-offender. The court heard that during the arrest, police searched two backpacks which contained two of the knives plus several items of clothing, neck gaiters and a handwritten pledge of "allegiance to the caliph."

The 18-year-old sat quietly as he was sentenced, then waved to his father as he was led from the court.

The court heard he was raised in a household supportive of Islamic State ideology. However, the judge rejected the man's claim that he was influenced or pressured to support that ideology by other people, saying his "return to extremist ideology was a product of his own volition."

The man will be eligible for parole in October 2028.