An investigation at Cambodia's UN-backed war crimes court has concluded there was no theft of documents from a Khmer Rouge leader's defence team, a spokesman said Wednesday.

Last week Michiel Pestman, Dutch defence lawyer for regime ideologue Nuon Chea, said he suspected confidential papers had been stolen from his office after he found them floating among waterlilies in a pond at the court.

But spokesman Lars Olsen said a tribunal security report had concluded there was no theft in the incident some local media dubbed "Waterlilygate".

"The main conclusions are clear: there is no evidence to substantiate allegations of foul play or theft of documents," Olsen told reporters at a press briefing.

Olsen said that details of exactly how the documents ended up in the pond would be revealed in the report.

Head of defence office Richard Rogers said Nuon Chea's defence had followed proper procedures to dispose of the documents, so methods must be improved to ensure confidentiality.

"If there are weaknesses in the system, I'm sure they'll be addressed by the (court's) security section," Rogers said.

The troubled tribunal, the Extraordinary Chambers in the Courts of Cambodia (ECCC) which is trying former Khmer Rouge prison chief Duch, also faces accusations of political interference by the government and claims that Cambodian staff were forced to pay kickbacks for their jobs.

Duch has stated that he took orders to kill and torture from Pestman's client Nuon Chea, who is commonly referred to as the Khmer Rouge "Brother Number Two."

Others in detention awaiting trial besides Nuon Chea are former head of state Khieu Samphan, foreign minister Ieng Sary and his wife and minister of social affairs Ieng Thirith.

Khmer Rouge leader Pol Pot died in 1998 before facing justice, and fears over the health of ageing suspects hang over the court.

The long-awaited first trial has heard Duch, whose real name is Kaing Guek Eav, acknowledge responsibility and beg forgiveness for overseeing the torture and execution of more than 15,000 people at the notorious Tuol Sleng prison.

Up to two million people were executed or died of starvation, disease and overwork as the Khmer Rouge movement emptied cities and enslaved the population on collective farms in its bid to create a communist utopia.