A mental competency hearing will be held in a Florida court for a 20-year-old accused of being an internet troll who attempted to organise terror attacks in Australia and the US.

Joshua Ryne Goldberg, while living at his parents' Florida home, allegedly had multiple fake online personalities, including an Islamic State Perth-based jihadist who went by the name "Australi Witness".

A prison psychologist who examined Goldberg has concluded he is not competent to stand trial.

In July, Goldberg allegedly told an FBI source, "I'm trying to get a mujahid in Melbourne to carry out jihad, but he keeps delaying it".

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

Goldberg is also accused of encouraging a man he thought was a terrorist to make a pressure cooker bomb and detonate it in a crowd filled with firefighters at a Kansas City 9/11 ceremony.

Goldberg also allegedly took responsibility for encouraging two armed men to attack a Muhammad drawing contest in Texas in May.

US District Court judge James Klindt will decide at a Florida hearing on December 14 whether Goldberg is mentally competent to stand trial.