"We aren't in a disaster yet, but we're on the brink of one," he said.
Cambodia is one of the world's poorest countries, with average incomes less than $300 a year and more than 40 per cent of the population living below the poverty line.
"There is a real risk that the opportunity to lift a whole generation out of poverty will be squandered"
The Global Witness report, entitled Country for Sale
, said researchers had found millions of dollars paid by oil and mining companies are missing from national accounts – money that, the group said, could eventually help Cambodia become independent of foreign development aid.
"The same political elite that pillaged the country's timber resources has now gained control of its mineral and petroleum wealth," Hayman said.
"Unless this is changed, there is a real risk that the opportunity to lift a whole generation out of poverty will be squandered."
The group said foreign aid donors should make further assistance conditional on Cambodia implementing new governance measures for key industries.
The report cites Australian mining giant BHP Billiton as confirming it paid one million dollars to the Cambodian government, but Global Witness says the funds have not been declared in the 2006 or 2007 government revenue reports.
It also alleges that $7.5 million paid to the Cambodian National Petroleum Authority by an Indonesian energy firm also did not reach the national treasury.
The allegations were denied by Suy Sem, the minister of Industry, Mines and Energy, who said Global Witness "always defames the government".
"Regarding the exploration for oil and gas, we operate under procedures that are very fair and transparent and based on the rule of law," he told The Associated Press news agency in the Cambodian capital, Phnom Penh.
In 2007 a Global Witness report Cambodia's Family Trees accused relatives of Hun Sen as well as close friends and senior government and military officials of stripping what remains of the Cambodia's forests for their own profit.
The government banned copies of the report from being distributed in Cambodia, although it can easily be downloaded from the internet.
Cambodia's officials at the time dismissed that report as "totally groundless, unacceptable rubbish", accusing Global Witness of engaging in a "political campaign" against the government.