An autistic Florida internet troll who posed online as Perth jihadist "Australi Witness" and told an FBI informant he was organising a terror attack in Melbourne has been jailed in the US for 10 years.

Joshua Ryne Goldberg, 22, lived with his parents in Florida, spent 14 to 20 hours a day online, suffered from depression and was "terrified by most human interactions", the US District Court in Jacksonville heard.

He became fixated on stirring up fears about terror attacks in Australia.

Goldberg created numerous bizarre online personas and reached out to Australian journalists at Fairfax and SBS and Perth-based reporters from 9 News, Channel 7 and Ten News with what he claimed was "inside information" about terrorists.

The FBI source, who engaged with him online after US and Australian authorities closed in, asked Goldberg: "Why Australia bro? Just curious. Are they as bad as the US?"

"And yes, Australia is bad as the US," Goldberg allegedly replied.

In another exchange Goldberg told the FBI informant he was planning a Melbourne terror attack.

"Right now, I'm trying to get a mujahid in Melbourne to carry out jihad, but he keeps delaying it," Goldberg told the FBI source.

"He has a gun already, but he insists on making a video message to the kuffar with a black shahada flag.

"He also insists on getting his friend to carry out jihad with him. I am afraid he'll get caught before he can do it."

Goldberg also went online to encourage the 2015 attack by two gunman on a Muhammad art exhibit and contest in Garland, Texas.

Goldberg also encouraged the FBI source, who Goldberg thought was a terrorist, to make a pressure cooker bomb and detonate it in a crowd at a Kansas City 9/11 ceremony.

US authorities raided Goldberg's parents home on September 10, 2015.

He entered a guilty plea to attempting to maliciously damage and destroy a building by means of an explosive for the Kansas City incident.

Prosecutors and Goldberg's lawyer both recommend he be sentenced to eight years' in prison.

Goldberg requested he serve the sentence at a federal medical prison facility in North Carolina.

Online aliases Goldberg used included: Junaid Thorne, an Australian radical Muslim who was used to attract real terrorists; Amina Blackberry, an African-American conservative woman living in Washington, DC; Michael Slay, a neo-Nazi calling for the extermination of white people; Emily Americana, a Samoan woman in Alaska who does not like fat people or Europeans; Nuke Europe, a European Jewish guy who experienced anti-Semitism throughout Europe; and Flappy Bird, a far left woman who promoted gender issues on behalf of women and transgender people.