(CNN) Australian filmmaker James Ricketson has been sentenced to six years in a Cambodian prison, after being found guilty of spying for an unnamed country.

The 69-year-old filmmaker was arrested in Cambodia in June 2017 after flying a drone over a rally organized by the Cambodia National Rescue Party, an opposition group that was later dissolved by the government.

A statement released by Ricketson's family Friday said they were "absolutely devastated" by the verdict and sentence.

"We are in utter shock at this outcome and that James, an innocent Australian, has been sentenced so harshly. Our family lives this tragedy daily," his family said.

Ricketson's lawyer Sam Onn Kong told CNN that Ricketson would seek a royal pardon, which could take up to a month to be decided.

Meanwhile his family urged the Australian government to intervene, saying that Ricketson's health had suffered during the 15 months he's spent in Prey Sar prison, in the Cambodian capital Phnom Penh.

"James is almost 70 and is not in good health, our family is very scared about what will happen to him in there if things take a turn for the worst," they said in the statement.

"We're not sure how long he can endure the conditions he is forced to live in in Prey Sar prison, where he is subject to cramped quarters, lack of sleep, subsistence food and close to zero medical care."

James Ricketson was accused of endangering national security by flying a drone over an opposition party rally last year.

The Australian government said it acknowledged the conviction and was providing full consular assistance.

"Mr Ricketson is subject to legal proceedings under Cambodian law and must now consider his response to the court's decision using the avenues open to him under Cambodian law," said the statement from Foreign Affairs minister Marise Payne.

"The Australian Government will consider what further appropriate support we can provide after that time."

HRW: 'Ridiculously excessive charges'

Before the verdict was announced Friday, Kong said that the prosecution had failed to provide any concrete evidence of espionage -- or even name the country that he was accused of spying for.

"Which words, which sentences, which paragraphs effect national defense?" Kong asked the prosecutor at Ricketson's trial, according to Australia's national broadcaster ABC.

"Which country is James spying for? Is it Australia? Is it America? Please let us know."

Advocacy group Human Rights Watch condemned the Cambodian judicial system in a statement following the verdict Friday.

"This trial exposed everything that's wrong with the Cambodian judicial system: ridiculously excessive charges, prosecutors with little or no evidence, and judges carrying out political orders from the government rather than ruling based on what happens in court," Phil Robertson, Deputy Asia Director, Human Rights Watch said.

"When it comes to a conviction in a Cambodian court, clearly no facts are required."

CNN has attempted to contact the Cambodian government for comment but hasn't received a response.

The Australian Directors' Guild (ADG) called on the Australian government to intervene on Ricketson's behalf.

"We call on the new foreign minister, Marise Payne, to contact her counterpart in Cambodia and seek clemency for James and for him to be sent home" ADG CEO Kingston Anderson said in a statement.

"Based on the evidence and what we know of James we do not believe he was spying for anyone. He was in the wrong place at the wrong time and may have contravened some local regulations but he was not spying for any government.

The Australian government has not commented.

Veteran filmmaker

Ricketson has traveled back and forth between Cambodia and Australia for more than 20 years, filming documentaries about life in the Southeast Asian country.

His son, Jesse Ricketson told CNN that his father's first trip to Cambodia was to film "Sleeping with Cambodia," a 55-minute documentary described by Screen Australia as an "exploration of paedophilia and children at risk."

Since then he has set up charitable ventures in the country, including fostering a Cambodian girl, and her mother.

The ABC reports that Ricketson had been critical of Cambodian leader Hun Sen, and that Cambodian media sympathetic to the government had labeled him an "important spy" acting to foment a "color revolution."

The prosecution's main legal arguments focused on emails, which were retrieved after his arrest, including one message to former Australian Prime Minister Malcolm Turnbull, urging him to withdraw an invitation for Hun Sen to visit Australia in 2016, Kong said.

Ricketson said in his closing statements at the trial that the "notion that a spy would provide national security information to a prime minister via a Gmail account is both fanciful and ludicrous," according to the ABC.

Ricketson's son Jesse told CNN Thursday that months of detention in a Cambodian prison had taken a toll on his father's physical and mental health.

"(It's been) hard for us, most specifically James. He's an old man and not in the best health -- conditions in prison are very hard. There's also a human toll on all those who support him."

He said his father had never meant to offend the Cambodian people or government.

"He's an outspoken guy who has a lot of opinions but he's always come from a place of love for Cambodia -- he loves the people, loves the place," Jesse Ricketson said.