More than a third of the planet’s natural world heritage sites face significant threats such as invasive species, logging and poaching, and climate change is a looming menace to prized ecosystems, according to a major new assessment.

The first ever analysis of all 228 natural world heritage sites found that 21% have a good conservation outlook, with 42% deemed to be “good with some concerns”.

However, 29% have “significant concerns” and a further 8% are listed as “critical”, which means they are deemed to be “severely threatened” and require urgent attention to avoid their natural value being lost.

The IUCN World Heritage Outlook, released at the World Parks Congress in Sydney, found that 54% of world heritage sites are well managed, but 13% are seriously deficient in protecting species and landscapes.

The report cites invasive species, the impact of tourism, poaching, dams and logging as the most pressing threats, although climate change may soon eclipse all of these factors.

“In terms of current threats, the most pressing is invasive species but climate change is the most serious potential threat,” said Elena Osipova, world heritage monitoring officer at the IUCN. “We’ve already seen the impact of climate change and the problem is that climate change can increase the impact from other threats.”

Most of the 19 critically threatened world heritage sites are in Africa, including the Virunga national park, which contains around half of the world’s remaining mountain gorillas. The Everglades national park in Florida is also on the critical list, mainly due to the area’s declining water quality, introduced pest species and vulnerability to climate change.

Three key Australian sites are listed as being a significant concern – the Great Barrier Reef, Kakadu and Queensland’s wet tropics. The declining state of the Great Barrier Reef has become a contentious political issue in Australia, with the government agreeing to a Unesco plea to stop dumping sediment in the ecosystem’s waters.

Machu Picchu in Peru, Tanzania’s Serengeti and the Sundarbans national park in India, home to an endangered population of tigers, are also considered to be of significant concern.

The best-ranked world heritage sites, with few threats, include Australia’s Lord Howe island, Mount Etna in Italy and the Dorset and east Devon coast in the UK.

Cyril Kormos, vice chair of the IUCN world commission on protected areas, said the assessment wasn’t intended to be political but should help countries manage world heritage sites better.

“This is something we all need to ensure the success of,” he said. “If we fail to protect the most valuable, iconic protected areas on the planet, we fail as a conservation community. “

A separate report unveiled at the World Parks Congress, a once-a-decade conservation event organised by the IUCN, found that the world is broadly on track to meet targets on the expansion of protected areas.

The United Nations Environment Program’s Protected Planet report found that 15.4% of the planet’s land and inland water areas and 3.4% of oceans are now formally protected. A total of 6.1m square kilometres has been placed under protection since 2010 – an area almost the size of Australia.

Targets agreed by more than 190 countries state that at least 17% of the world’s terrestrial areas and 10% of its oceans must be protected by 2020.

The report states that while this target is likely to be met, other problems present themselves. Specifically, many protected areas are poorly managed, aren’t located in areas of important biodiversity and aren’t well connected, which means animals and plants can’t spread and flourish.

Conservation officials at the congress also pointed out that the 17% target was essentially a political one, with scientists advocating up to 50% of the world’s surface to be protected in order to save threatened species and safeguard critical habitat that provides water.

Achim Steiner, executive director of UNEP, said there needed to be a greater appreciation of the economic benefits of protected areas, citing a Finnish study that found that for every euro invested in nature, the community benefited by 10 euros.

“We look at these things as a cost to the taxpayer, without looking at the multiplier in the economy,” he said. “The dividing line between private and public funding is very anachronistic. Most of the forests in Europe are under private ownership, for example.

“The private sector is not just multinational mining enterprises; some can be important co-investors. They are an under-utilised and under-appreciated contributor to how to finance protected areas in the future.”