Sprawling Jakarta is home to nearly 30 million people. (File)

Abandoning Jakarta for a new capital in Borneo won't save the fast-sinking Indonesian megacity from disaster and could even spark a fresh environmental crisis in a region home to rainforests and endangered orangutans, critics have warned.

President Joko Widodo announced this week that the Southeast Asian nation's political heart would be moved nearly 2,000 kilometres (1,200 miles) east to a yet-to-be-built model city as a way of easing pressure on densely populated Java.

Sprawling Jakarta -- home to nearly 30 million people counting its greater metropolitan area -- is plagued by a host of ills, from eye-watering traffic jams and pollution to the risk of earthquakes and floods.

It is also among the fastest-sinking cities on the planet.

Experts say one-third of Jakarta could be submerged by 2050 due to excessive depletion of groundwater reserves, rising sea levels, and volatile weather patterns driven by climate change.

Construction of the new capital at the eastern edge of Borneo -- the exact location remains a mystery -- is set to begin next year with some 1.5 million civil servants expected to start moving by 2024, at a cost of 466 trillion rupiah ($33 billion).

Known as Kalimantan, Indonesia's section of Borneo, which it shares with Malaysia and Brunei, is home to major mining activities as well as rainforests, and is one of the few places on Earth with orangutans in their natural habitat.

But mining and palm oil plantations are already threatening Borneo's environment and endangered species habitats, a problem that could only get worse if a big city is built near a key conservation area, green groups say.

Cautious optimism

The area was the scene of a major oil spill last year.

"East Kalimantan is already under heavy environmental strain," said Zenzi Suhadi, a spokesman for Indonesian environmental network WALHI.

"There are hundreds of mines and plantations. When the area gets overstressed (with a new capital) are they going to move again to another place?"

Indigenous leaders in Borneo expressed cautious optimism that the new capital would help marginalised groups and supply an economic boost to a region with about 20 million residents -- a small chunk of the archipelago's 260 million people.

"But we're also worried," said Yulius Yohanes, an academic and community leader for the Dayak ethnic group.

"The Dayaks are concerned about the condition of our forests -- we've always had a deep connection with them."

Indonesia's government has pledged to invest billions of dollars to fix Jakarta's problems and said that protected forests would not be affected by the new capital.

Widodo recently announced a permanent moratorium on issuing new permits to clear primary forests for agricultural plantations and logging.

"But there is still a risk that primary forests could be affected," Suhadi said.

As blazes devastate the Amazon rainforest, Indonesia has deployed thousands to battle annual forest fires -- often triggered by burning for land-clearance purposes -- that are ravaging vast swathes of jungle and belching out toxic fumes in Borneo and Sumatra island.

Massive fires in Indonesia caused a smog outbreak across Southeast Asia in 2015 that set off diplomatic frictions with neighbouring Malaysia and Singapore -- and may have caused more than 100,000 premature deaths, according to a US study.

"The plan to move the capital should be called off if the government can't fix the fires issue," said urban planning expert Nirwono Joga.

In the overcrowded capital, the plan to move has been met with scepticism by many residents.

A recent survey found some 95 percent of those polled were against the move, and some poked fun at the plan online -- mocking the country's leader and suggesting the new capital could be called "Jokograd" or "Saint Jokoburg" in a nod to his outsized ambitions.