Bangkok: Accused Australian spy James Ricketson has pleaded to be released from Cambodia's harshest jail so he can continue the charity work for Phnom Penh's poor which he has been doing for more than 20 years.

The 68-year-old Sydney filmmaker says that after almost six months in Prey Sar prison on the outskirts of Phnom Penh "I am still confused as to what I have done other than flying a drone without a permit to deserve such punishment".

In this photo from Cambodian website Fresh News, Australian filmmaker James Ricketson is seen apparently operating a drone.

"I hope that any misunderstanding that may exist can be resolved so that I can continue with my humanitarian work and to being papa to my adored Cambodian family."

Ricketson's Australian family members say he is in declining health in the prison and there are "grave fears" for his life unless he is released soon.