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".

"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.

They say he has high blood pressure which is un-medicated and is suffering edema, swelling of the legs, and skin irritations including rashes, lice and scabies.

Lawyers have appealed to Cambodia's Supreme Court to overturn a lower court's rejection of a bail application which has left Ricketson in jail while prosecutors prepare a brief of evidence to be presented to judges.

Ricketson was confronted by more than a dozen police, clapped in handcuffs and carried by the arms and legs from Cambodia's waterfront in June shortly after visiting a rubbish dump where he was providing aid to scavengers.

Earlier that day he was photographed flying a drone over a rally by supporters of the opposition Cambodian National Rescue Party (CNRP) which has been at the centre of a sweeping crackdown on political freedoms in the country ruled by strongman Hun Sen for more than three decades.

Government-aligned media outlets in Phnom Penh have accused Ricketson of being an "important spy" and linked him to a supposed conspiracy to topple the Hun Sen regime in an Arab-Spring-style uprising.

Analysts, diplomats and human rights groups say Hun Sen, a former commander of the murderous Khmer Rouge, has concocted the claim to justify a campaign to destroy the opposition and silence his critics ahead of elections next year.

Authorities charged Ricketson with collecting material with a view to supplying them to a "foreign state or their agents". This suggests Cambodian authorities will allege he was spying for Australia, which has a controversial agreement to send refugees from Nauru and Manus Island the country.

No details of the allegations against Ricketson have been made public.

Authorities are believed to be translating into Khmer 16,000 emails found on Ricketson's computer.

Ricketson detailed his charity work in a message from jail, saying "as a filmmaker, journalist and philanthropist, I am distraught the work I do in Cambodia is construed as anything other than altruistic".

Cambodia's poor featured in his 1997 film Sleeping with Cambodia, and for 21 years he has been documenting the life of a street beggar who he adopted as his daughter.

"I am also currently trying to raise around $140,000 to buy houses for 18 families working in the Phnom Penh rubbish dump," he said.

"My efforts to help this small community are well documented," he said, adding at the time of his arrest he was financing the setting up of a shop for a couple who had been scavenging in the rubbish dump.

Ricketson is a controversial figure in Phnom Penh where he has campaigned for the release of a British man jailed in Prey Sar on child sex charges whom he insists is innocent, and has waged a campaign against fellow Australian Scott Neeson, founder of the Cambodian Children's Fund, who last year published "fact sheets" to counter the allegations.

Last year Ricketson was found guilty of defaming an organisation that investigates foreign paedophiles, and received a suspended sentence.

In Australia, Mr Ricketson has been in a long-running dispute with Screen Australia.

He won an Australian Film Institute award for Best Screenplay Adaption for his 1995 film Blackfellas, which he also directed.

Ricketson faces up to 10 years jail if convicted on spying charges.