Cambodia's Supreme Court has upheld a criminal defamation conviction against a former Adelaide school teacher and ordered him to pay $A1.2 million in fines to the Cambodian People's Party led by Prime Minister Hun Sen.

The ruling exhausts legal avenues for Son Chhay, vice-president of the opposition Candlelight Party and a dual Australian and Cambodian citizen who was awarded an Order of Australia in 2010.

It also means he will not be able to contest national elections in July.

"Yes I was told by my lawyer that the Supreme Court upheld the conviction late on Thursday, Son Chhay told AAP.

"I was at a hearing a week before but I was not there for verdict and I don't feel any different than when they took me to court in the first place.

"When a case is prosecuted by the ruling party we don't expect anything but a conviction," the 67-year-old politician said.

"We will approach the Australian embassy next week. I need further advice about what to do next."

Son Chhay's Candlelight Party was the best-performing opposition party at local commune elections in June, winning more than 22 per cent of the popular vote.

But he was sued after he said publicly: "These election results do not reflect the will of the people who were intimidated. Their votes were bought and stolen."

The ruling was issued amid a long running crackdown on dissent, the forced closure of independent news outlet Voice of Democracy and long-running court battles against the Candlelight Party which was forged out of the remnants of the Cambodian National Rescue Party (CNRP).

The CNRP was dissolved by the courts in late 2017, enabling the CPP to win all 125 seats contested in the national election a year later. More than 100 CNRP supporters have since been convicted and jailed in four mass trials, many in absentia, for incitement and plotting to overthrow Hun Sen.

Among them was Hong Lim, a retired member of the Victorian parliament who was acquitted in October 2021. Late last year another court ordered Son Chhay's two homes to be seized and used as security to ensure damages are paid to the CPP and NEC.

"The Australian embassy has been attending the hearing and they have done a good job and been quite supportive," Son Chhay said.

"Only a liberal democracy can promote peace and stability."

Cambodia is ranked in second last place on the World Justice Project (WJP) 2022 Rule of Law Index at 139 out of 140 countries, one place behind Afghanistan and one place above Venezuela.