And the long-serving former MP, who has been in Jakarta to lobby the Indonesian government to speak out about Cambodian Prime Minister Hun Sen's violations of human rights and curtailing of democratic norms, has a message for the world: don't send international observers to the July 29 national election.

Rainsy hasn't given up all hope of MPs from his opposition Cambodian National Rescue Party - more than 100 have been arrested or exiled since September 2017 - being about to stand in the July 29 elections against Hun Sen's government.

But time is rapidly running out.

"Anything can happen until the last minute, so I remain hopeful that pressure is building up on Hun Sen to make him reverse course, to back track. We want Hun Sen to organise real elections. A real election means an election with the participation of the opposition parties," he told Fairfax Media.

He predicts an "electoral sham" if the CNRP is not, as appears likely, allowed to participate.

"We don’t want any international observers to be sent to Cambodia if there is no participation by the opposition. The sending of observers should be conditional [on this]. For observers, it would be meaningless to observe an electoral sham with a foregone conclusion. Such observers would only endorse a fake election."

If the voter participation rate below 70 per cent, Rainsy said, and the opposition party was not allowed to stand it would be a slap in the face for the ruling party.

Thus far, Russia is one of the few countries that has committed to sending monitors to observe the conduct of what Cambodia's government has suggested will be "free and fair" elections.

According to Rainsy, Hun Sen - the strongman ruler of Cambodia since 1997 - had jeopardised stability, democracy and the country's independence through his close relationship with China.

"Siding with China at any cost means that Cambodia is losing her independence. Hun Sen does not respect democratic principles, he only needs support from China. And by siding with China, Hun Sen is playing with fire, he is breaking up the solidarity of ASEAN countries," he said.

"By siding with China, giving military facilities to China, Hun Sen is disrupting the balance of power in the region. This will threaten the security and interests of many countries in the region."

Last month, China and Cambodia held a 17 day military exercises on Cambodian soil, with hundreds of soldiers and military hardware involved.

At the same time, Cambodia has in the last year signed a series of development deals with China, while welcoming the influx of Chinese infrastructure cash and tourists to offset its deteriorating relationship with and investment from the west.

The increasingly close relationship between China and Cambodia has also led to questions being raised about Cambodia playing a spoiler role in the consensus-focused ASEAN bloc.

Rainsy accused Hun Sen of "weakening ASEAN to the extent of preventing any joint statement that should be firm enough to uphold the rule of law on the South China Sea", as an example.

Finally, the exiled opposition leader had a message for Australia, Indonesia and Japan - three of the signatories to the 1991 Paris Peace accords, that brought the ruinous Cambodian civil war to an end.

This trio of countries, he said, should speak out about the ongoing abuses of human rights and democracy in Cambodia.