Cambodia's opposition party is calling for targeted sanctions and a suspension of technical aid, saying democracy is dead in the South-East Asian nation which hosts a small number of refugees from Nauru.

Key points: Mu Sochua said she was tipped off about plans to arrest her next week

Mu Sochua said she was tipped off about plans to arrest her next week Prime Minister Hun Sen has gone on the attack since losing ground in a local election last year

Prime Minister Hun Sen has gone on the attack since losing ground in a local election last year Critics accuse Canberra of keeping quiet about Cambodia's slide into authoritarianism

For years, the ruling Cambodian People's Party (CPP) has attacked the opposition in the streets and the courts, but now the Cambodian National Rescue Party (CNRP) president is locked up and senior members have fled.

The latest to leave is high-profile politician Mu Sochua.

"The word flee is the word that I've always wanted to keep out of my vocabulary, because fleeing is giving up," Mu Sochua said.

"But in this situation [fleeing] is saving the voice, keeping that voice for outside."

Mu Sochua said she was tipped off about plans to arrest her next week.

Prime Minister Hun Sen has gone on the attack since losing ground in a local election last year.

In recent speeches, he called opposition leaders "rebels" and "spies", accusing the party of receiving CIA backing to topple his regime, which he has ruled for 32 years.

Mu Sochua said the political landscape had now changed, with the increasing threats making it impossible for the opposition to operate inside Cambodia.

"Now that we are fighting for democracy from outside, we will continue to speak to the international community very frankly, and demand actions," Mu Sochua said.

"This time for statements has passed. Now, it's time for sanctions, targeted sanctions. Also suspension of technical aid to the Government of Cambodia."

The ABC has requested a comment from Australia's Department of Foreign Affairs and Trade about sanctions and suspending technical assistance.

Cambodian Prime Minister Hun Sen is the world's sixth longest-serving premier. ( Supplied: The Phnom Penh Post )

'Democracy still ongoing, ruling party says

Australia is a donor country to Cambodia, recently giving Hun Sen a bonus $40 million of aid money in exchange for resettling a handful of refugees from Nauru.

Critics accuse Canberra of keeping quiet about Cambodia's slide into authoritarianism due to the refugee deal.

Australia's investment in Cambodian democracy goes back to the early 1990s — with Australia playing a leading role in the $2 billion United Nations intervention that tried to bring peace and elections after the murderous Khmer Rouge regime.

A spokesman for the ruling party said calls for sanctions were "not proper" and rejected allegations Cambodia was no longer a democracy.

"The democracy is still ongoing and multiple political parties still exist," Phay Siphan, a spokesman for the Council of Ministers, said.

However, the CNRP is the only other major political force, with just one seat going to a third party in local elections.

Phay Siphan said the campaign against the opposition was not driven by fear of an election defeat.

He told the ABC support for his party increased in the most recent election, but the National Election Commission website shows the CPP's tally of commune seats fell from 97 per cent in 2012 to 71 per cent in 2014, with the raw vote fairly evenly split at 50.76 (CPP) to 43.83 (CNRP).

A national election is due in July 2018.

Hun Sen has made it clear he will not hand over power easily, repeatedly threatening "civil war" if he is voted out.