More than 5m hectares (12.35m acres) of customary-owned land in resource-rich Papua New Guinea have been signed over to unrepresentative landowner companies and foreign-owned corporations for up to 99 years, according to a report by Greenpeace.

Of the total 5.1m hectares covered by special agricultural and business leases (SABLs), 75%, or 3.9m hectares, are controlled by foreign-owned companies under 54 subleases or development agreements. Malaysian and Australian firms control at least 3m hectares through 32 SABLs.

PNG has the world’s third largest tropical forest, but demand for its logs has led to extensive deforestation. A satellite study in 2008 said the forests of this south Pacific country were being chopped down so quickly that more than half of its trees could be lost by 2021.

The 5.1m hectares of customary-owned land represent 11% of the country and more than 16% of accessible commercial forests. PNG log exports grew by almost a fifth in 2011, largely due to logging under SABLs. Since 2006, logging companies have exported more than 1.5 cubic metres of whole logs, netting $145m (£92m) for the mostly Malaysian companies involved. Almost all the logs were exported to China.

The Greenpeace report, Up for Grabs, is highly critical of the previous government of Sir Michael Somare for allocating forests to industrial logging companies, which often occured against the wishes of people who live in PNG’s forests and customary landholders.

“The previous Somare government continued this predatory relationship with customary landholders by actively facilitating the granting of SABLs with legislative amendments that enabled logging companies to gain easy access to customary-held forested land,” said Greenpeace.

In May 2011, the PNG government announced a commission of inquiry into SABLs following international condemnation. The commission completed its inquiry in May this year, but will not be made public until it is tabled in parliament by the newly elected prime minister this year.

Last week, PNG’s rival prime ministers ended a political feud that had left the country with two leaders for most of the past year. Somare, the elder statesman of South Pacific politics at 76 and the country’s first prime minister in 1975, recontested his seat despite being ill for much of last year. Peter O’Neill was voted in as prime minister after Somare was ruled ineligible due to his prolonged absence from parliament.

Although O’Neill had the support of parliament, the supreme court twice ruled that Somare was the legitimate prime minister, leaving the country with rival leaders. Last week’s agreement means O’Neill is likely to head the new government and form a coalition with backing from Somare. O’Neill’s People’s National Congress party is expected to win most of the seats in parliament – 3,500 candidates stood for 100 seats. Votes are still being counted.

Despite PNG’s mineral wealth, successive governments have been unable to deliver infrastructure or services to a country of 6.5 million people, with about 80% of the population living on subsistence village farming and small cash crops. The general elections were PNG’s eighth since independence from Australia in 1975.

Greenpeace said O’Neill’s leadership could be a turning point in PNG’s land policy. “He is a progressive figure and is best placed to implement the findings of the commission of inquiry,” said Paul Winn, author of the report. “But he’s had to team up with Somare’s party, with vested interests, so he might find it difficult to implement the recommendations in full.” Winn said the commission had done a thorough job. “We believe it is a hard-hitting report, saying how elites have benefited from corruption.”

The Greenpeace report said the single biggest issue highlighted during the commission’s inquiry was the lack of fair representation of customary landholders in agreeing to SABLs being granted over their land. The report pointed out that the Department of Lands and Physical Planning, the agency responsible for evaluating and granting SABL applications and registering subleases, was described by judicial authorities as grossly incompetent and entirely corrupt. In many cases, said Greenpeace, it was the corporations applying for logging or agricultural development that financed the government approval process.

To address many of the underlying issues that led to PNG’s “land grab”, Winn said it was vital for the new government to seek international help – possibly from Norway, Japan and Australia – to develop a national land planning process to identify land to be used for development, conservation or tourism and to ensure that land use benefited all of the population.