Cambodia's Information Minister has been seen giving local journalists $US200 just minutes after voting in an election labelled a "sham" by rights groups.

Khieu Kanharith cast his ballot at Toul Kork Primary School in Phnom Penh.

"I want to give these guys (local media) some money but you guys (ABC) are here," he said.

About 15 minutes later, he walked to his car and the ABC witnessed him hand over two $US100 bills, which were later distributed to Cambodian reporters.

The ABC confirmed that reporters and camera operators received $US20 each.

Sorry, this video has expired Cambodian Information Minister bribes local media at ballot box.

By chance, another senior official to vote at the school was Om Yen Tieng, the head of the country's Anti-Corruption Unit.

He said Khieu Kanharith's cash gift to media was not corruption.

"Up to our law, I'm [sic] think not, nothing to ask on this," he said.

Sunday's Cambodian election has been described as "fraudulent" because the main opposition party was made illegal last year and its leader thrown in jail on treason charges.

The campaign period has also been marred by threats, intimidation and bribery, with officials from the ruling Cambodian People's Party giving cash and gifts to voters in return for attending rallies and promising to vote.

The banned opposition has called on Cambodians to boycott the polls.

Voting is not mandatory in the election, but authorities have warned that anyone who boycotts the vote will be seen as a "traitor".

Cambodia is ranked 161 out of 180 countries in Transparency International's 2018 corruption index.

PM voted this morning

Hun Sen cast his ballot at a polling station in Takhmua in Kandal province, southeast of Phnom Penh. ( AP: Heng Sinith )

Prime Minister Hun Sen cast his vote at Takhmao commune in Kandal province, about nine kilometres from the capital.

He showed off his inked finger but declining to speak to reporters.

Earlier exiled former opposition leader Sam Rainsy used Facebook to urge Cambodians not be intimidated.

"Don't be intimidated by the dictator who wants to force you to go to vote," he said in a video which has been viewed more than 420,000 times.

Some 8 million people are registered to vote, and the National Election Committee is expected to announce the preliminary results on Sunday night.

The official results are expected in mid-August.