Australia will not send diplomats to monitor the Cambodian election, which has been described as a "sham".

The United States has taken a similar stance, staying away from the polls to avoid legitimising an election that is almost guaranteed to continue Prime Minister Hun Sen's 33-year reign.

The Cambodian People's Party (CPP) is running without major challenge, after the main opposition party was made illegal last year, its leader jailed and its MPs banned from politics for five years.

While 19 small parties have registered for Sunday's vote, all are either linked to the CPP or are too tiny to aspire to more than a seat or two.

The Cambodian People's Party is running without any major opposition. ( ABC News: Liam Cochrane )

Threats, intimidation, bribery and the shutdown of independent media have marred the lead-up to the vote.

"Reflecting our concerns with the electoral process, Australia will not monitor the national election," a spokesperson for Australia's Department of Foreign Affairs and Trade said.

"We will continue to raise our concerns at the highest levels of the Cambodian Government."

The decision was welcomed by the now-defunct opposition.

Threats, intimidation, bribery and the shutdown of independent media have marred the lead up to the vote. ( ABC News: Liam Cochrane )

"I'm delighted and I thank Australia — a democratic country — for supporting us and taking our side," Thach Seitha, a former senator for the outlawed Cambodian National Rescue Party (CNRP), said.

The response from Hun Sen's ruling party was frosty.

"Only the Cambodian people can judge this election, the outsiders' assessment doesn't reflect the will of the Cambodian people and could also be political," CPP spokesman Sok Ey San said.

The ruling Cambodian People's Party has mustered tens of thousands of monitors but there will be few independents.

The ruling party has raised tens of thousands of monitors — many under the youth wing of Hun Sen's son Hun Many — but it is estimated as many as half of the polling stations will not have an independent monitor.

The National Election Commission is stacked with officials considered loyal to the ruling party.

Opposition figures have called for a boycott of the election, a movement that's become known as the "clean finger" campaign — a reference to the indelible ink used to mark the index finger of voters.

Cambodia's now-outlawed opposition party CNRP has accused the government of buying votes. ( Supplied: Cambodian National Rescue Party )

"This election is a fraud and a lot of our supporters have shown pictures of their clean fingers on Facebook, but we don't know how successful the boycott will be," Thach Seitha said.

"Many of those supporters live in the countryside and they live in fear because of intimidation from the local authorities."

This week five former CNRP officials in Battambang province were fined $3,300 for posting photos on social media showing their clean fingers and encouraging people not to vote.

As of Friday, Cambodian internet providers appeared to be blocking access to the most popular independent Khmer-language news sites, including Radio Free Asia, Voice of America and Voice of Democracy.