The Australian architect of a peace plan for Cambodia and an exiled opposition MP have chastised the Turnbull government for failing to take the Hun Sen regime to task as the country's dream for democracy crumbles.
Former foreign minister Gareth Evans, who helped bring peace to war-torn Cambodia in the early 1990s, said watching Cambodian people vote in the 1993 poll was one of the proudest moments in his life.
The country is now on a spiral towards authoritarianism following last year's ban on the main opposition party, the arrest of opposition leader Kem Sokha and forced the closure of an English-language newspaper.
The opposition party had been poised to strongly challenge his rule at the July election.
Professor Evans condemned Australia's limp reaction.
"Australia hasn't been willing to say boo to the goose," he told AAP in Canberra.
The Australian National University chancellor said the "manifestly indefensible" refugee resettlement deal had compromised Australia's response to the Cambodian situation.
But next week's special summit of Australian and south-east Asian leaders presented Prime Minister Malcolm Turnbull with an opportunity to rectify Australia's silence.
Prof Evans said Mr Turnbull needed to be frank when he sits down for a bilateral meeting with Cambodian prime minister.
"Cambodian government has been getting away with murder," Prof Evans said.
"It's time for Cambodia's political leaders to be named, shamed, investigated and sanctioned by the international community - that remains my opinion today."
Prof Evans described being on the receiving end of a Hun Sen tirade and said there was a fair chance Mr Turnbull will also face a blast.
"It came in spades - a withering, ground-glass-eyed, menacing, finger pointing, denunciation of me. It went on for a full 15 minutes," he said.
Exiled Cambodian opposition MP Mu Sochua urged Australia to step up and play a leadership role.
"(Australia) has left a legacy in Cambodia," Ms Sochua told an audience at the Australian National University in Canberra.
"Use it to put the hope of democracy back on track."
She called for targeted sanctions against the Hun Sen regime.
Ms Sochua, was forced to flee Phnom Penh last October.
In the lead up to the Association of South East Asian Nations (ASEAN) and Australia summit in Sydney, Hun Sen threatened to beat up protesters for burning his effigy.
Hundreds of Cambodian expats are expected to protest during the prime minister's visit to Australia.
Victorian state MP Hong Lim condemned Australia's ongoing defence cooperation with Cambodia, amid reports up to eight protesting villagers had been shot dead by the Khmer military in a land dispute in Kratie province.
"We need to end it now. We can't be in any way complicit in these atrocities," he said.
Australian filmmaker James Ricketson is in jail in Cambodia, facing spy charges after flying a drone over a political protest in the capital Phnom Penh last June.
He faces five to 10 years behind bars if convicted.
Exiled former Cambodian opposition leader Sam Rainsy believes Mr Ricketson's arrest was designed to frighten and deter foreign journalists.
Australian Associated Press