Amid bushfires raging across Australia and as the international community struggles to contain fires in the Amazon, Indonesia's neighbours are again complaining of transboundary haze from forest fires on the islands of Sumatra and Borneo.

Key points: 2015 forest fires in Indonesia were the worst for two decades

2015 forest fires in Indonesia were the worst for two decades Toxic smog threatens a diplomatic row with Indonesia's neighbours

Toxic smog threatens a diplomatic row with Indonesia's neighbours Environmentalists warn the fires could continue to worsen

There have been more than 73,000 fire alerts across Indonesia since the beginning of September alone, causing flight cancellations and closures of schools.

A state of emergency has been declared across at least six Indonesian provinces and police announced Monday that they had arrested almost 200 people in relation to the fires.

As toxic smog blankets Singapore and Kuala Lumpur, Malaysia's Government has openly criticised Indonesia for failing to prevent fires.

But with Indonesia's meteorological agency warning that dry conditions will continue until October, some worry the disaster could get even worse.

Why are the fires burning?

As in the case of Brazil's Amazon rainforest, many of the current blazes in Indonesia have been deliberately lit as a cheap way to clear land for agricultural use.

"The fires which would be occurring in Indonesia are part of a process called deforestation fires, where fires are being lit in tropical rainforest or in secondary forest to clear land, as a process for small-scale and possibly large-scale agriculture," David Bowman, Professor of Pyro-geography and Fire Science at the University of Tasmania, recently told the ABC.

"It can be illegal burning, it can be mass burning to try to clear vegetation to establish plantations like palm oil."

Many of the fires are deliberately lit to clear land for agriculture, including palm oil plantations. ( Reuters: Willy Kurniawan )

Indonesia is the world's largest producer of palm oil — much of which comes from Sumatra and Kalimantan.

While recent years had seen a decline in the severity of fires and haze, the return of El Niño in 2019 signals a longer, hotter dry season. As a result, this year's fires are the country's worst since 2015.

Up to 70 per cent of fires that occurred that year occurred on degraded peatlands, releasing an enormous amount of carbon dioxide into the atmosphere.

What is the impact of forest fires?

Indonesia's palm oil-producing provinces are worst impacted by peat fires. ( Greenpeace: Ulet Ifansasti )

The burning of peatland and forests has major public health and environmental consequences.

During September and October of 2015, the carbon dioxide emissions produced by fires in Indonesia exceeded daily emissions from the entire US economy — an economy 20 times the size of Indonesia's.

The 2015 fires caused more than 100,000 deaths across South-East Asia, according to a study from Harvard and Columbia University scientists.

"Sight in places where the smoke is really thick is about 300 to 500 metres," Ratri Kusomohartono, a campaigner for Greenpeace who has just returned from Central Kalimantan, told the ABC.

"It looks like it's going to be as bad as 2015."

Palangkaraya, the capital of Central Kalimantan once tipped as Indonesia's new national capital, saw the air quality index (AQI) reach 2,000 on Sunday evening.

An AQI rating of more than 300 is considered hazardous.

Air pollution in Kalimantan has been recorded at extreme levels. ( Supplied: AirVisual )

Beyond devastating public health impacts, the fires also have huge consequences for Indonesia's natural environment. According to data from the World Bank, Indonesia's forest cover has dropped by nearly a quarter since 1990.

"Humans are expanding into these environments that weren't traditionally burnt very often," Mr Bowman added.

"The Indonesian archipelago — particularly Borneo — is a vast, basically fire-free environment. Because of human actions, clearing and burning, fire activity is increasing and causing the loss of the rainforest."

Deforestation causes habitat loss, posing a grave threat to the future of various species such as orangutans, tigers and sun bears.

Indonesia's orangutans are critically endangered due to habitat destruction and illegal hunting. ( ABC News: Adam Harvey )

Why are Malaysia and Singapore unhappy?

As in past years, 2019 fires have been mostly concentrated in the palm oil-producing provinces of Central and West Kalimantan, and Jambi and Riau in Sumatra — those geographically closest to Malaysia and Singapore.

While pollution levels measured there haven't been as extreme as in Indonesia, people from Singapore to Kuala Lumpur have felt the effects.

President Joko Widodo in August said he was "embarrassed" to visit Malaysia and Singapore, given fires were already impacting them with haze.

Malaysia has criticised its neighbour over failure to prevent the smog. ( AP: Vincent Thian )

Malaysia's Environment Minister Yeo Bee Yin has been vocal in her criticism of Jakarta's failure to prevent fires in 2019.

But Ms Yeo's Indonesian counterpart Siti Nurbaya Bakar has denied the existence of transboundary haze and claimed pollution is stemming from fires in Malaysia too.

"Minister Siti Nurbaya should not be in denial," Minister Yeo responded in a post on her Facebook page.

Singapore's National Environment Agency, meanwhile, reported over the weekend that haze had reached its worst levels in three years.

"The return of the haze is a reminder of the seriousness of the problem, which has affected the ASEAN region for years," said the country's Environment Minister Masagos Zulkifli in a statement.

Singaporeans were urged to stay indoors over the weekend. ( Reuters: Feline Lim )

"It both pollutes the air we breathe and emits greenhouse gases that cause climate change.

"This is why there is need for stronger resolve and cooperation … in order to achieve our vision of a haze-free ASEAN by 2020," Mr Zulkifli said.

What has Indonesia done to address the problem?

In the immediate term, Indonesia has deployed thousands of firefighters across Sumatra and Kalimantan, as well as thousands of volunteers.

The Government says dozens of helicopters have been mobilised for water bombing in Kalimantan.

Indonesian police have arrested some 185 people suspected of being involved in slash-and-burn practices.

"This is a last resort. The most important thing is prevention," National Police spokesman Dedi Prasetyo said.

After the 2015 fires, Indonesia vowed to prevent such a disaster from occurring again. It arrested a number of corporate executives in relation to the haze, and Government figures show that deforestation slightly declined in 2018 compared with 2017.

But critics say that large plantation companies have never properly been held accountable.

"The companies and Government blame communities," said Ms Kusomohartono.

Forest cover in Indonesia has dropped by nearly a quarter since 1990. ( Reuters: Nova Wahyudi, file photo )

"But what we heard from local communities was that people lighting the fires were paid by companies. For them it's pretty big money to burn hectares and hectares of land."

Then last month, President Widodo announced a permanent moratorium on deforestation with the declared aim of improving "primary forest and peatland governance."

But Greenpeace dismissed the announcement as mere "propaganda", claiming that "1 million hectares inside the moratorium area have burned between 2015–2018 as a result of forest fires."

What more could be done?

Observers say that weak law enforcement is the major barrier to preventing the burning of peat forests.

Having faced criticism from its neighbours, Indonesia last week announced it was clamping down against a handful of Malaysian and Singaporean companies it claims are responsible for fires in Riau and West Kalimantan.

In the meantime, Malaysia and Singapore have both offered their assistance to battle the blazes.

Schools have been closed in Sumatra and Kalimantan due to dangerous levels of air pollution. ( AP )

"The forest fires must be extinguished as soon as possible," said Malaysia's Environment Minister Yeo. "Malaysia has offered to provide any support needed to the Indonesian government."

"As always, we stand ready to help suppress the fires on the ground," Singapore's Environment Minister Zulkifli added.

But Ms Kusumahartono said that no matter how many resources were thrown at firefighting, the blazes were unlikely to stop until the wet season arrives in November.

"They're just saying we're waiting for heavy rain. It's just so hard to manually put them out."