Kamer Nizamdeen who was detained in a supermax Australian prison after he was accused of plotting to kill former Prime Minister Malcolm Turnbull and then Foreign Minister Julie Bishop has described the Australian Federal Police investigation into his case as " immature, unprofessional and irresponsible."

Charges against Nizamdeen were dropped late last month with police admitting he probably did not write threats contained in a notebook.

The 25-year-old, who was charged with creating a document in connection with preparing for a terrorist act,  spent four weeks behind bars. 

At a press conference in the Sri Lankan capital Colombo t he University of NSW contractor has slammed the AFP investigation as "embarrassing and biased".

"The method in which the AFP conducted themselves was completely immature, unprofessional, irresponsible, embarrassing, and biased to say the least," he said, according to the ABC.

The method in which the AFP conducted themselves was completely immature, unprofessional, irresponsible, embarrassing, and biased to say the least," he said, according to the ABC.

"I strongly believe this happened because I am an Asian, on a student visa.

"And the AFP... had the wrong impression that I did not have the resources or capability of defending my innocence.

"After I was charged and incarcerated, my only consolation was having faith in the Australian judicial system, which is completely independent to the Australian Federal Police.

"Following eight hours of questioning, detectives thought it was fit to charge me under Australian's draconian laws.

"This was after I denied several times that the handwriting was mine