Thousands of representatives from more than 160 nations meet in Australia this week at the once-a-decade World Parks Congress as scientists warn that countries are failing to care for protected areas.

The week-long summit will see up to 5,000 scientists, politicians, activists and business leaders converge in Sydney to lay out the global conservation agenda for the next 10 years.

The meeting follows an Australian-led scientific review last week that said governments needed to do more to protect national parks or risk losing their economic, environmental and social benefits.

"A lot of nations have backtracked on their commitments over the last five years," University of Queensland professor James Watson, lead author of the study published in the journal Nature, told AFP.

"And so the first thing is to put protected areas back on the table... and from that it's not just recognising the importance of protected areas, it's actually funding them."

Watson said allocating US$45 billion -- or 2.5 percent of global military spending -- to protected areas annually would see them adequately managed. The United Nations estimated in 2007 that annual spending on protected areas was about US$7 billion.

In the review, titled "The performance and potential of protected areas", scientists were critical of several developed and emerging economies in their treatment of such places over the past decade.

In the United States, home to the world's first protected site at Yosemite, the National Park Service had deferred maintenance of between US$9.03 billion and US$13.28 billion due to insufficient funding, the researchers said.

In West Africa, a 2014 review of parks managing lion populations found half with management plans had no money to implement them, the study added.

The scientists also highlighted measures taken by nations such as Indonesia, Brazil, Britain, Russia, Cambodia, Japan, and Australia that they said were leading to a decline in the effectiveness of protected areas.

- 'Less than quarter effectively managed' -

Since the first World Parks Congress in 1962, the conference has helped shape policy and provided technical guidance on managing protected areas, while presiding over an increase in the number of conservation sites.

An estimated 15 percent of the Earth's land and three percent of oceans are defined as protected. Some of the best known include the Grand Canyon in the US, Machu Picchu in Peru and Tanzania's Serengeti.

Despite the increase, the scientists said some studies showed less than one quarter were effectively managed and were facing growing additional pressures from climate change and poaching.

"Some of the most iconic protected areas, such as Ecuador's Galapagos National Park, are undergoing significant degradation, partly due to an inability to manage them effectively," said co-author Marc Hockings, also from the University of Queensland.

The researchers cited a report by the World Wildlife Fund (WWF) conservation group in January which found 543 instances of protected areas in 57 developing countries being downgraded, downsized or removed from conservation over the past 50 years, with industrial activities or local land claims the primary causes.

Host nation Australia has itself come under scrutiny from the United Nations Educational, Scientific and Cultural Organization (UNESCO) in recent months over concerns about the health of the Great Barrier Reef.

Environment Minister Greg Hunt has sought to shore up his country's conservation credentials, and will host an Asia-Pacific Rainforest Summit on Wednesday ahead of the World Parks Congress.

Watson said he hoped the Congress, from November 12-19, would work towards accounting for the value of protected areas -- not just as tourism sites but also as places that conserve biodiversity and serve as sources of food, water and shelter for people.

"The 33 largest cities in the world get 50 percent or more of their water from protected areas. If the protected areas weren't there, these cities would not exist," he said.

"We've got to have an accounting system that gets on the books and talks about why (the protected areas) are actually important... and that (they) help our economies and make us prosperous."