As expected, Prime Minister Hun Sen's Cambodian People's Party won Sunday's election in a landslide, extending his 33-year rule in a result described as marking the death of democracy in Cambodia.

The main opposition party, the Cambodian National Rescue Party, was banned last November and its leader, Kem Sokha, now languishes in jail and the party's former leader Sam Rainsy, and deputy leader Mu Sochua have gone into exile.

While official election results will not be released until August, an official from Hun Sen's CPP said the party had won overwhelmingly, with an estimated 100 seats in the 125-member parliament.

A pro-government news website, DAP, later estimated the government may have won 114 seats.

The United States and European Union have harshly criticised the conduct of the election in Cambodia, raising the prospect of sanctions and an expansion of visa restrictions for government officials.

White House Press Secretary Sarah Sanders said in a statement that Sunday's vote "failed to represent the will of the Cambodian people" and was a "significant setback ... to the democratic system enshrined in Cambodia's constitution".

Like the US, the EU and Japan, Australia declined to send election monitors, and Foreign Minister Julie Bishop expressed serious concern the poll "has reversed more than 25 years of progress towards democracy in Cambodia".

"As a longstanding friend of Cambodia, Australia will continue to urge the Cambodian Government to take steps to allow free and open political debate without violence and intimidation," she said. There was no mention of visa bans or sanctions.

Monovithya Kem, daughter of the jailed Kem Sokha and who serves as the deputy Director for Foreign Affairs of the CNRP, said Australia's response was disappointing.

"We are quite disappointed to have not seen the Australian government taking any action at all. If you look at the free world, whether it is the EU or the US, they have at least taking some sort of measure against what has been happening since September," she told Fairfax Media on Monday.

"We are calling on the Australian government to take some sort of action. Number one, you could place a visa ban on Cambodian officials that have been found violating human rights and democratic principles in Cambodia. That has been implemented by the US already. We are counting on Australia to do the same."

"And also, Australia being a hub of Hun Sen’s elite – it’s like a gang hub – there is a lot going on in Australia, the Australian government needs to look into it. We are hoping the Australian government will finally take some action."

Earlier this month, an Al Jazeera investigation examined in detail the property and business interests of Hun Sen's family and Cambodian government and military officials in Australia.

Mu Sochua said on Monday that July 29, 2018, would be remembered as the day Hun Sen’s regime had invited sanctions on Cambodia.

She called on Indonesia and France to call together all the parties to the 1991 Paris Peace deal, which ended decades of war and civil strife in Cambodia, to address the "dangerously escalating crisis". She also called on the ASEAN nations to condemn the election's conduct and result.

A defiant Mu Sochua insisted her party, the CNRP “cannot and will not be destroyed by a politically motivated ruling of an unjust court".

"It’s leaders and martyrs may be imprisoned and exiled but will never be silenced. CNRP is, and will continue to be, the party of the majority of Cambodians who believe in a free and open society."

China's influence in Cambodia's domestic politics has grown massively in the past decade, and Hun Sen is increasingly reliant on his country's giant northern neighbour for investment and tourism.

Mu Sochua said China had played a major role in propping up Hun Sen but that the influential neighbour needed to be respectful of Cambodian sovereignty.

"It is imperative that any development, any investment in Cambodia respects the rule of law, human rights, democracy and the will of our people. That it respects our environment ... we ask that China be a friend of the people of Cambodia, that China be a friend of democracy and human rights.