"I have a right to free speech under the Cambodian constitution," 68-year-old Ricketson said as guards led him from the country's Supreme Court.

"I would like to think the Australian government would defend my right to free speech," he said.

Ricketson arrived at the court almost an hour after the delay was announced in an apparent jail transfer mix-up.

"I'd love to know what country I am supposed to be spying for," Ricketson told Fairfax Media while handcuffed to another prisoner.

Court officials said the case was delayed until January 31 because authorities were late bringing Ricketson from jail for Wednesday's hearing.

Ricketson said he was not confident of being released on bail because it would be a "loss of face" for those building a case against him.

Authorities are investigating Ricketson over his alleged links to a now-disbanded opposition party, which has been accused of attempting to overthrow strongman Hun Sen in a purported United States-backed conspiracy.

He was arrested after flying a drone over a rally on Phnom Penh's riverfront staged by the opposition Cambodian National Rescue Party in June and accused of spying against the government.

The opposition party's leaders have been jailed, are in hiding or have fled the country in a sweeping crackdown on opponents of Hun Sen ahead of elections scheduled for mid-year.

Analysts say the supposed conspiracy has provided Hun Sen, one of the world's most notorious autocrats, with an excuse to target his political opponents, as he shrugs off any pretence of democracy in the country where Australia has a deal to send refugees from Nauru.

Opposition figures and the US have strongly denied involvement in any conspiracy.

Ricketson, a prolific letter writer and blogger and award-winning documentary maker from Sydney, was a familiar figure over years at opposition and protest rallies in Phnom Penh, where he has been filming a documentary on a former street beggar he has supported for decades.

Ricketson told an earlier court hearing he came to Cambodia "to help poor people and make films, not to be a spy".

For years he has supported scavengers at a rubbish dump on the outskirts of Phnom Penh while writing critical blogs about some non-government-organisations in Phnom Penh and campaigning against the conviction of a convicted British child sex offender, who he says is innocent.

Ricketson has been held in pre-trial detention since June in Prey Sar, one of Cambodia's notoriously harsh jails, as he protests his innocence.

"I am still confused as to what I have done other than flying a drone without a permit to deserve such punishment," Ricketson wrote from a cell he is sharing with 27 other prisoners.

The circumstances of his arrest and detention have been murky.

Officials said he has been accused of spying "for a foreign state or agents" but provided no further details.

Fresh News, a pro-government news site, accused Ricketson of being an "important spy" and linked him to the supposed plot to overthrow Hun Sen that allegedly involved opposition leaders, staff of NGOs, US embassy officials and journalists.

Support for Ricketson is growing in Australia where thousands of people have signed a petition calling for his release and criticising the Turnbull government for failing to intervene in his case.

Australian journalist Peter Greste, a press freedom advocate who was jailed along with two other Al Jazeera journalists in Egypt, has thrown his support behind the campaign.

Greste tweeted to his 50,000 followers: "Help free another journalist in prison on national security charges. No evidence that James Ricketson in Cambodia is guilty of anything other than caring."

People who know Ricketson say any suggestion he is was spying is ludicrous.

Ricketson is suffering un-medicated high blood pressure and other ailments and his family fear he may die in jail.

Foreign Minister Julie Bishop said the "Australian Government is continuing to provide consular support, while ensuring we do not prejudice in any way his current situation."