Australian filmmaker James Ricketson has been transferred to a prison hospital in Cambodia with various health complaints as he awaits trial on espionage charges.

Key points: Jess Ricketson claims his father has lost 20 per cent of his body weight

Jess Ricketson claims his father has lost 20 per cent of his body weight James Ricketson described a series of doctor no-shows and incompetent consultations

James Ricketson described a series of doctor no-shows and incompetent consultations The Cambodian Government provided no evidence of who he's alleged to have spied for

James Ricketson is 69 years old and his months of detention in a cell with 126 other men are taking their toll.

In a letter to the Australian ambassador in Cambodia, Mr Ricketson said he had chest pains and coughing fits and had been x-rayed for tuberculosis. But he hasn't been told the result of the test.

Mr Ricketson has been moved to a health centre within the prison where he can at least lie down, but there is limited medical treatment available.

He was arrested in June, but so far the Cambodian Government has provided no evidence of who he is alleged to have spied for.

His son is in Phnom Penh and regularly visits his father in Prey Sar prison on the outskirts of the capital.

"The main complaint is he's got a chest infection of some sort, which is not properly diagnosed at this point but could be tuberculosis or pneumonia, we don't really know," Jesse Ricketson said.

James Ricketson was sent to a hospital in Phnom Penh for a chest X-ray but was returned to prison without a diagnosis or further consultation.

"He's also got a variety of skin irritations … swollen lymph glands, he's suffering a lot of loss of energy, not really sleeping properly, he's lost 20 per cent of his body weight," Jesse Ricketson told the ABC.

Cambodia's prisons are notoriously overcrowded and have extremely limited medical facilities.

"They basically don't have the capacity to diagnose or treat anything, what he is getting there is a bed that he can lie down on … proper rest," Jesse Ricketson said.

Ricketson pleads with the Australian embassy for help

Sorry, this video has expired James Ricketson arrives at the Phnom Penh Municipal Court in February to answer questions about espionage charges.

In the letter to Australia's ambassador to Cambodia, Angele Corcoran, James Ricketson described his conditions and urged officials to provide more assistance.

"[The] chest pains … could relate to my heart or be purely [musculoskeletal] and related to my age and sleeping on the concrete floor with only two folded towels and cotton blankets as a 'mattress," he wrote, in a letter dated April 22 and posted on a Facebook page run by his adopted daughter Roxanne.

He was scathing about his medical treatment, describing a series of doctor no-shows and incompetent consultations.

James Ricketson has been charged with spying. ( Supplied )

"My 'full medical check up' on 13th April did not include having my temperature taken, my pulse rate measured … a blood test, a medical examination or a conversation with a doctor," James Ricketson wrote.

"This exercise in medical futility cost me $US300 ($399).

"Could the Australian embassy please, on my behalf, contact Dr Hung and ask him what the X-ray suggested was vis-a-vis my illness and what medical steps should be taken to address the problem," James Ricketson wrote in his letter to the ambassador.

The embassy in Phnom Penh has previously said it is providing standard consular assistance.

Foreign Minister Julie Bishop wrote to her Cambodian counterpart earlier this year but the contents of the letter have not been made public.

James Ricketson was arrested for flying a drone over a political rally. ( Supplied )

The Australian filmmaker was arrested on the streets of Phnom Penh in June, a day after flying a drone above a political rally.

He has been charged with collecting information for a foreign state that could prejudice Cambodia's national defence.

The prosecution has not indicated who Mr Ricketson is alleged to have been spying for.

The investigation has so far focused on emails about and to the opposition Cambodian National Rescue Party, which was outlawed last year by the country's authoritarian Prime Minister Hun Sen.

Mr Ricketson denies all wrongdoing, saying his relationship with the opposition was purely journalistic.

Long pre-trial detention is taking its toll

James Ricketson is known for his charity work. ( Facebook: James Ricketson )

Australia reporter Peter Greste — who was jailed in Egypt — has called for James Ricketson to be released.

"Help free another journalist in prison on national security charges," Mr Greste tweeted in December.

"No evidence that James Ricketson in Cambodia is guilty of anything other than caring," he said.

James Ricketson has been visiting Cambodia for 20 years, helping poor families and making documentaries.

Under Cambodian law, an initial pre-trial detention of up to six months can be extended twice.

James Ricketson's son said the long pre-trial detention was taking its toll.

"He's a strong man and he's hanging in there but it's definitely wearing him down," Jesse Ricketson said.

"He's 69-years-old, we're moving into the hot season … and not having proper medical treatment is kind of a disaster, we're really worried about what could happen."