The United States, Japan, western European nations and Australia – which has largely avoided criticising Hun Sen's regime in recent years – all declined to send election monitors to observe this poll and there were widespread reports of intimidation and threats on Sunday to avoid voter turn out dipping below 50 per cent.

Indeed, on Friday Hun Sen warned "traitors" of consequences if people boycotted the election, despite voting not being compulsory in Cambodia.

Hours after the polls closed, official government figures suggested turnout in the poll had reached 82.17 per cent, or about 6.8 million people – suggesting calls for a boycott from the banned and disbanded Cambodian National Rescue Party, the only credible opposition, had failed as turnout had risen from 68.5 per cent in 2013.

Preliminary results released on Sunday evening, on a province by province basis, saw the ruling party's share of the vote hovering at around the 75 per cent mark.

Twenty parties, including Hun Sen's CPP, contested the poll and the government was counting on a high voter turnout to bestow a thin veneer of legitimacy on the election. But many of those parties are just a few months old and have links to the ruling regime.

The expected victory for the CPP once again confirms Cambodia's drift towards China and further entrenches authoritarian rule, 27 years after the Paris peace accords of 1991 – in which Australia played a key role – brought decades of war and civil strife to an end and attempted to set the nation on a course towards multi-party democracy.

Griffith University Southeast Asia politics lecturer Lee Morgenbesser told Fairfax Media that the regime's narrative had been that a higher turnout meant the poll was more credible but that "if you look at North Korea, Syria, or Azerbaijan for example, higher turnout is not a marker of legitimacy, it’s a mark of a lack of competition".

"This is the end of the road for democracy in Cambodia, though I’ve thought the country has been authoritarian for years. This is also a moment of reckoning for Australian foreign policy with Cambodia, and the idea that we can procure benefits like the refugee deal, without any collateral damage. Australia needs to re-think its relationship with Cambodia," he said.

"Cambodia is already a client state of China, it has been for some time. Australia doesn’t have the influence to stop that.  So the Australian government needs to be on the same side of the fence as the US."

Last week, the US House of Representatives passed the Cambodia Democracy Act 2018. If confirmed by the Senate, it will clear the way for sanctions on Hun Sen and his regime for undermining democracy and committing human rights violations.

Human Rights Watch's deputy director in Asia, Phil Robertson, said the reality was that "regardless of turnout, the election is not genuine as defined in article 25 of the International Convention on Civil and Political Rights ratified by Cambodia because of dissolution of CNRP".

And Sam Rainsy, the former leader of the CNRP who narrowly missed out on victory in 2013 and who is now in exile, said the government's win was a hollow one.

"He [Hun Sen] had dissolved the only party that could have beaten him, the CNRP, in advance."

"The participation rate announced by the authorities does not reflect reality."

Surrounded by bodyguards, election officials and media, Hun Sen and his wife Bun Rany were all smiles as they voted on Sunday morning at a teacher training school about 15 kilometres south of Phnom Penh.

After voting, he paused outside to greet election observers, then announced to the media: “By law I cannot make any comment.”

Hun Sen is a former Khmer Rouge commander who defected from Pol Pot's murderous regime and allied himself with Vietnam, which was the de facto ruler of the country for ten years until 1989.

Cambodian Sun Sina, 39, who cast his ballot in the Kandal Steung district of Kandal province, told Fairfax Media on polling day that he expected "my party [the CPP] to win the election, surely".

"Before the elections in the past there were a lot of people, it was crowded and messy," he said.

Kandal provincial Governor Mao Phirun insisted voters were not being intimidated and that "the citizens are happy".

"I would like to convey my message to my compatriots living abroad that Cambodia sticks to the liberal, multiparty democratic regime."

The polling station chief in Kandal Steung, Vanna Im, said: "Almost everyone has voted except for the sick and the dead."

Cambodia's National Election Commission will announce the official results of the poll on August 15.

with Reuters