The son of an Australian filmmaker detained in Cambodia for 12 months without trial says now is a "critical juncture" for the Australian Government to put diplomatic pressure on the Hun Sen regime.

James Ricketson was arrested on the evening of June 3, 2017 and charged with espionage.

He has now spent a year in pre-trial detention, waiting for a Cambodian judge to investigate his case.

"The evidence has been gathered, the investigation has now been closed and so we are at this critical juncture now where the Cambodian legal system can show us that justice can be done," Mr Ricketson's son, Jesse, told the ABC's 7.30 in an exclusive interview.

Jesse Ricketson has uprooted his life in Sydney and moved to Phnom Penh to be able to visit his father in prison.

Jesse Ricketson has moved to Cambodia so he can visit his father James Ricketson in prison. ( Supplied: Jesse Ricketson )

"My father's health is not good in there. He's squished into a tiny cell with a lot of people," Jesse Ricketson said.

James Ricketson, 69, is currently in the prison hospital due to a chest complaint and various skin problems.

But the medical treatment is extremely limited and any serious illness could be "a disaster", his son said.

"Worst case scenario is he could die in there," Jesse Ricketson said.

He regularly makes the 40-minute trip to Prey Sar prison, taking food, money and books.

But the ordeal is taking its toll on his father's mental state.

"I think it's really hard for him in there. He doesn't speak much of the local language, just basic words, and there aren't a lot of English speakers, so in terms of just his mental health he's very much isolated and alone."

'100 per cent not a spy'

James Ricketson speaks to the media at the Supreme Court in Phnom Penh in January. ( Reuters: Samrang Pring )

James Ricketson has been visiting Cambodia for more than 20 years, making documentaries and helping impoverished Cambodians.

"One of my dad's best qualities is that he cares about people," Jesse Ricketson said.

"He sees them struggling to survive, struggling to make money, even to feed their own children, and so he can't just see that and go back to Australia. He's drawn to do everything he can to help."

Jesse Ricketson, 36, dismisses the espionage charge that carries a possible 10-year sentence.

"My father is not a spy, 100 per cent categorically not a spy, and there's no evidence that supports the charge against him," he said.

James Ricketson's incarceration comes at a tense time in Cambodia, with an election next month.

Prime Minister Hun Sen has banned the opposition party, jailed its leader and has threatened civil war if his party is not re-elected.

It seems James Ricketson's strong opinions online and his interest in the opposition has put him in the crosshairs.

James Ricketson with a family in Cambodia. ( Supplied: Jesse Ricketson )

"They've looked at emails in which he expresses opinions about Cambodian politics but that's really all, it's not anything more malicious than that," Jesse Ricketson said.

The ABC has previously obtained some of the emails being investigated, which appear to show normal journalistic communications and some spirited opinions in favour of the opposition party — but nothing suggesting espionage.

"There was absolutely no evidence that he was involved in anything he was charged with," said Naly Pilorge, director of the Cambodian human rights organisation LICADHO.

"It's shameful that a person at that age would be held in pre-trial detention for that long."

Ms Pilorge said lengthy pre-trial detentions were an unfortunate reality in Cambodia.

'Robust' diplomatic representations needed

One Australian who has been through a similar ordeal is journalist Peter Greste, who spent 400 days imprisoned in Egypt.

James Ricketson has been involved in charity work in Cambodia. ( Facebook: James Ricketson )

"I think it's shocking. We haven't seen any clear evidence yet that James is guilty of any offence that justifies this kind of imprisonment," Mr Greste told 7.30.

"I feel for him. I know what it's like to be stuck in prison for such a long time with an uncertain future.

"The Australian Government and Julie Bishop have been very keen to take a very prominent role in the Human Rights Council, talking about freedom of the press and other human rights issues.

"And if it wants to be taken seriously, it needs to step up and make sure that it follows through on those kinds of principles when it comes to those states over which we have leverage."

In handwritten letters — often written at night when other inmates are sleeping — James Ricketson has complained that the Australian Government isn't doing enough to help.

Australia's Foreign Minister Julie Bishop rejected the accusations.

"I have raised Mr Ricketson's case at the highest levels, including by writing to my Cambodian counterpart Minister Prak Sokhonn," Ms Bishop said.

"The Cambodian Government has confirmed receipt of that letter."

She said Mr Ricketson's case was raised with the Cambodian Government at the ASEAN-Australia Special Summit in March.

"There are limits on what we can do as the Australian Government cannot intervene in Cambodian legal proceedings, just as foreign governments cannot intervene in Australian legal proceedings."

James Ricketson wants the Australian Government to do more to help him. ( Supplied: Jesse Ricketson )

While James Ricketson's frustration is evident in the reams of letters penned to Prime Minister Malcolm Turnbull, Ms Bishop and Australia's ambassador in Phnom Penh Angela Corcoran, his son is trying to see the positive potential of diplomatic pressure.

"We very much welcome diplomatic representations being made by Julie Bishop and DFAT in this case," Jesse Ricketson said.

"We need them and now is definitely the time where we would hope to see more of that — very robust, strong representation on James' behalf.

"He's an innocent Australian locked up abroad. We need their help and we do trust that they [the Australian Government] will be doing everything they possibly can."