Mr Ricketson has been charged with gathering information that could jeopardise Cambodia's national security. He could face up to 10 years jail if convicted.

Australia's standard response in matters where its citizens get into legal trouble overseas is that it cannot intervene in the legal affairs of another nation. It provides only consular assistance.

But Mr Ricketson was charged on Friday with "receiving or collecting information, processes, objects, documents, computerised data or files with a view to supplying them to a foreign state or its agents, which are liable to prejudice the national defence".

Authorities in Phnom Penh have released scant details about the allegations against Mr Ricketson after a Cambodian media outlet published photographs of him flying a drone over a political rally.

But the nature of the charge suggests the authorities will allege Mr Ricketson was spying for Australia or its agents, which would drag Canberra into the case.

Since Australia signed a $55 million refugee deal with Cambodia in 2014, Coalition ministers have developed close ties with the Cambodian regime, despite the fact its strongman Hun Sen has presided over more than 30 years of corruption, brutality and a readiness to subvert popular will. The UN refugee agency, Australian opposition parties and human rights and refugee advocates condemned the deal which has seen only a handful of refugees arrive in Cambodia from Nauru.

Confirmation of the charge against Mr Ricketson came after days of conflicting reports about his fate, including that he would be only be charged with staying in Cambodia illegally.

He was held for six days before being charged, which is well beyond the 72 hours allowed under Cambodian law, and has been denied visits by friends.

Cambodian authorities cracked down on the country's media and opposition parties ahead of local elections on June 4, including lodging court complaints against several journalists for seemingly routine reporting.

Non-government organisations were warned the government was closely watching them for any "bias" towards opposition parties.

Mr Ricketson, from Sydney, was flying the drone over a riverfront rally organised by the opposition Cambodia National Rescue Party.

For several years he has been working on a documentary about the party's former leader Sam Rainsy, who has been forced into exile to escape charges widely seen as politically motivated.

Mr Ricketson studied at the Australian Film and Television School and has made several award-winning features and documentaries.

Mr Ricketson is a frequent long-stay visitor to Cambodia where he is known to donate food and other help to impoverished families, including rubbish dump scavengers.

He has campaigned for the release of children from orphanages and for the release of a British man convicted and jailed over child sex who he says is innocent. In 2014 a Phnom Penh court gave Mr Ricketson a two-year suspended sentence for allegedly threatening to defame a refuge for girls run by an Australian charity.

He had been campaigning for the release of two girls, whose mother wanted them to come home, from the centre.

The Australian embassy in Phnom Penh said Mr Ricketson is receiving consular assistance but declined to comment further, citing privacy obligations.

Mr Ricketson could not be reached for comment.