The number of people killed defending the environment has doubled in the past 15 years, and corruption is driving the high rate, according to new research.

Key points: More than 1,500 people have been killed while defending the environment since 2002

More than 1,500 people have been killed while defending the environment since 2002 Corruption strongest predicter where people will be killed

Corruption strongest predicter where people will be killed Actual death rates probably higher than reported figures

Between 2002 and 2017, at least 1,558 people globally were killed while trying to defend land, forest, water or other natural resources from development.

Indigenous people were disproportionately represented among the dead, often defending land they lived on or relied on for resources.

Others killed included lawyers, journalists, rangers, community activists and leaders.

The research, published in Nature, is based on figures from environmental NGO Global Witness, and researchers said it was unclear if the increasing rate of killings reflected a surge of violence or an increase in reporting.

Newly released documents show that another government agency, as well as the Australian Federal Police, was involved in the investigation that led to the raid on the ABC in June.

The findings come after gold miners in Brazil fatally stabbed an Indigenous leader from a remote village in the country's north last week, according to officials.

The people killed during the research period were defending natural resources against deforestation for agriculture, mining, water access, and forestry logging.

But protesting any one of those activities versus another did not significantly correlate with a person's chance of being killed, said lead researcher Nathalie Butt from the University of Queensland.

Instead, the primary determinant was the level of corruption in the country where the protest took place, she said.

"Although the natural resources are what the conflict is over, that's not actually what is causing the violence. [It's] when there's corruption involved as well that's the key factor," Dr Butt said.

"People doing the murders might be the police or the military or people employed by the companies that are extracting the resources.

"In Brazil in 2017, there was a massacre of 10 land defenders and that was done by the police."

Actual deaths likely to be much higher

Brazilian President Jair Bolsonaro's hard-line rhetoric has prompted fears of more clashes between miners and Indigenous people. ( Getty Images: Carl de Souza )

The researchers based their corruption data on the World Justice Project Corruption Index.

What compounds the issue is that countries with the worst instances of corruption and higher death rate of environmental activists or defenders also tend to have low rates of murder conviction, Dr Butt said.

"In terms of conviction rates for murders, globally it's about 43 per cent, whereas for environmental defenders the average is about 10 per cent," she said.

"In some places it's even lower than that. So for instance, in Colombia, it's about 1 per cent."

Brazil and the Philippines top the list for the number of people killed for environmental activism, with 609 and 192 people slain in those countries respectively since 2002.

Brazilian President Jair Bolsonaro has been outspoken in his criticism of Indigenous land reserves in Brazil, raising fears that conflict between mining interests and environmental defenders will escalate.

Actual deaths of environmental defenders are likely to be under-reported in countries without a free press. ( Reuters: Muhammad Yamin )

Mr Bolsonaro has previously vowed to open Indigenous reserves to mining interests, and said he is looking for opportunities to partner with companies from the US and other nations to extract more mineral resources in Brazil.

While it is difficult to say whether the doubling of the death rate of environmental activists is due to increased reporting or actual increased violence, researchers said the global rate of killings is likely to be higher than reported.

Violence in Indonesia's West Papua, for example, is likely to be significantly under-reported due to the restriction of press freedom.

But while the high death rate tells part of the story, there is more to the suppression of environmental defenders than murder statistics, according to Keith Barney, a natural resource policy expert from the ANU.

Recognising the context and process that surrounds political violence against environmental defenders is important to understanding the whole picture, Dr Barney said.

Often a prominent community member or activist will be murdered or "disappeared" without a trace, with the intention of sending a message to others to keep their heads down.

"[With] specific instances of violence — everyone pays attention to it in small countries like where I work in Laos," Dr Barney said.

"For example, if there was a decision to 'disappear' one prominent civil society leader, that has the effect of silencing other people. That happened in Laos, with the state-linked enforced disappearance of Sombath Somophone."

It also happened in Cambodia, where prominent land and environmental defender Chut Wutty was shot dead by Cambodia's military police, according to Global Witness.

Supply chain transparency

Actual death rates are likely to be higher in places where press freedoms are limited, like Indonesia. ( Getty Images: Sutanta Aditya )

In order to curb some of the violence being perpetrated against environmental activists, companies need to have more transparency in their supply chains, the authors argue.

"Businesses, investors and national governments at both ends of the chain of violence need to be more accountable," they wrote.

Groups like FSC certify timber that has been sustainably sourced. ( Getty Images: Picture Alliance )

And there is some merit to that argument, according to forest and ecosystem scientist Rod Keenan from the University of Melbourne.

"Transparency, as they flag, is really important, so that companies are both conscious and taking action," he said.

"Conscious of where this might be occurring in their supply chains, and there's pressure on them to take action against it."

Professor Keenan added that consumers could play a role by ensuring they purchased items with a forest product certification from groups like FSC.

"They're a good way of demonstrating that human rights and the rule of law are being observed in the production of forest products."

But while supply-chain transparency is a good start, it's not a one-stop fix, Professor Keenan said.

"I think [the research] presents a model that it's largely big, greedy multi-nationals out there taking resources off local people," he said.

"But it's often the marginalised local people who are wanting access to environmental assets in protected areas, and rangers are impacted."

While the causes of conflict differ between regions, a context of post-colonial disenfranchisement and negative attitudes to Indigenous people often creates a context for conflict to occur, he said.

"Poaching, clearing land illegally for agriculture ... there's a range of different types of conflict going on and they're not all related to international supply chains."

"There's all these types of colonisation and settlement-type developments over hundreds of years that have led to these sort of situations."

Dr Barney agreed, and said looking at supply chains was just one way of analysing the problem.

"Violence against environmental defenders is not only about western supply chains," he said.

"Often there's a richer social and political context that needs to be understood."