Cambodia's exiled former opposition leader believes the Turnbull government hasn't done enough to push for the release of an Australian filmmaker held there because it is compromised over the refugee deal struck with the Hun Sen regime.
James Ricketson was arrested on spy charges last June after flying a drone over a political protest in the capital Phnom Penh.
He faces five to 10 years behind bars if convicted.
Emails between Mr Ricketson and self-exiled former opposition leader Sam Rainsy have formed part of the legal case against the Australian.
Cambodia is in the midst of steady democratic decline, with the country's Supreme Court dissolving Mr Rainsy's Cambodia National Rescue Party last year and jailing opposition leader Kem Sokha.
The opposition was poised to strongly challenge the 32-year rule of Prime Minister Hun Sen at the July election.
Mr Rainsy said Mr Ricketson was a "scapegoat".
"He's been held hostage to frighten other foreign journalists and to deter them from writing critical reports about the Hun Sen regime," he told reporters at the National Press Club in Canberra on Thursday.
Mr Rainsy said emails between the two were of the same nature of correspondence he receives from other foreign journalists.
Foreign Minister Julie Bishop this month made formal diplomatic representations to her Cambodian counterpart Prak Sokhonn on behalf of Mr Ricketson.
This followed 68,800 people signing a petition online urging Ms Bishop to intervene in the case.
"I'm surprised the Australian government does not do more to secure his release," Mr Rainsy said.
He pointed to how vocal the French government had been when a Cambodian-French dual citizen senator in his party, Hong Sok Hour, was arrested on dubious charges in 2015.
"The French government intervened because he was a French citizen and he was released," Mr Rainsy said.
He said failure to speak firmly gave the impression a citizen had been abandoned and would, therefore, be left to languish in jail for many years.
Asked why he thought Australia hadn't been as forward as the French, in Mr Ricketson's case, Mr Rainsy replied: "Because of the refugee issue."
"Canberra does not want to have any problem with Hun Sen," he said.
"Hun Sen feels there is total impunity for him."
Cambodia agreed in 2014 to resettle refugees from Nauru in exchange for a $40 million aid sweetener.
A further $15 million had been allocated for running costs.
Only seven people took up the offer and it's believed only three remain there.