He also been charged with preparing to enter a foreign country to engage in hostile activities after he allegedly made an earlier attempt to travel to Syria.
Last year Khaja hit the headlines when he was kicked out of high school for reportedly preaching Islamic State-style extremism in the playground.
Australian police say Tamim Khaja was looking for targets ahead of an imminent lone attack in Sydney.
Their extradition from Queensland has been approved and they are due to appear in a Melbourne court tomorrow.
Police allege he had been scouting sites in Sydney for an attack and trying to acquire a gun.
The 18-year-old has been taken into custody and charged with one count of acts in preparation for, or planning, a terrorist act.
"The planning was occurring now and we would say that an attack was probably imminent", New South Wales Police deputy commissioner Catherine Burn said. She would not identify the others and how they were linked, but said the teenager was acting alone in this alleged plot.
The teen has been under surveillance since he allegedly tried to board an global flight from Sydney Airport in February to join the Islamic State terrorist group in Syria.
Officers said Khaja had been known to police for about a year and had attempted to leave the country to fight with terror organisations overseas in February, but was unable to do so.
Tamim Khaja was arrested at his Macquarie Park home on Tuesday morning by counter-terrorism police. If convicted, he could face a maximum sentence of life in prison.
An Australian teen was on Tuesday arrested on charges of plotting a terror attack in Sydney and attempting to stage incursions overseas.
She said it was unfortunate that the group of people involved in these foiled attacks are getting "younger and younger".
Australian counter-terrorism police on Tuesday carried out raids across Melbourne connected with five men accused of planning to travel to Syria to join Islamic State via a journey that would start with a motor boat trip from Australia to Indonesia.
"As we all know, youth are vulnerable, particularly around the (radicalised)", Burn said, urging Australian parents and the community to be vigilant about their child's behavior.