Bali's airport will re-open after a nearly three-day shutdown, Indonesian airport authorities said, as ash from a volcano shifted direction. Picture: AP Photo/Firdia Lisnawati.

Denpasar - Bali's international airport will re-open Wednesday afternoon after a nearly three-day shutdown, Indonesian airport authorities said, as ash from a rumbling volcano that looms over the island paradise shifted direction.

The eagerly awaited decision opens up a window of hope for some of more than 120 000 tourists stranded after a spike in activity at Mount Agung grounded hundreds of flights since Monday.

But airport officials cautioned that the only direct international gateway to the tropical island could be shuttered again if winds change direction and towering columns of smoke and ash pose a risk to flights.

"The airspace will be re-opened" at 3:00 pm local time (0700 GMT), Bali Ngurah Rai airport spokesperson Arie Ahsanurrohim told AFP.

But "we are going to constantly monitor the situation on the ground," he added.

Ash is dangerous for planes as it makes runways slippery and can be sucked into their engines.

Mount Agung could produce a major eruption at any moment, officials have warned.

Read more: Bali warns of imminent 'bigger' Mount Agung eruption

Tens of thousands have already fled their homes around the volcano -- which last erupted in 1963, killing around 1 600 people -- but as many as 100 000 will likely be forced to leave, disaster agency officials have said.

Experts said Agung's recent activity matches the build-up to the earlier disaster, which ejected enough debris -- about a billion tonnes -- to lower global average temperatures by around 0.3 degrees Celsius for roughly a year.

"Small eruptions have been happening continuously but there's still the possibility of a bigger, explosive eruption," said I Gede Suantika, a senior volcanologist at Indonesia's volcanology agency.

"Activity remains high and we are still on the highest alert level."

Roadside signs that read "Volcanic danger zone. No entry!" underscored the potential risks of staying behind.

There is a 10 kilometre exclusion zone around Agung, which is 75 kilometres away from the beachside tourist hub of Kuta. 

As of Wednesday around 440 flights had been cancelled since the start of the week.

The airport on nearby Lombok island -- also a popular tourist destination -- has opened and closed several times in the past few days. It is currently open.

'Ring of Fire'

Some 100 buses are hauling visitors to several destinations including Indonesia's second-biggest city Surabaya -- 13 hours' drive and a ferry ride away -- and the capital Jakarta, the airport said, as torrential rain lashed the beach paradise.

The majority of Bali's tourists are Chinese, followed by Australians, Indians, British visitors and Japanese, according to the immigration office, which added that nearly 25 000 foreigners live on the small, Hindu-dominated island.

Also read: 'South Africans travelling to Bali must exercise extreme caution’

Foreigners whose visitor visas are expiring will be given a special permit to stay longer due to the eruption, the agency added.

Agung rumbled back to life in September, forcing the evacuation of 140 000 people living nearby. Its activity decreased in late October and many returned to their homes. 

However, on Saturday the mountain sent smoke up into the air for the second time in a week in what volcanologists call a phreatic eruption -- caused by the heating and expansion of groundwater. 

So-called cold lava flows have also appeared -- similar to mud flows and often a prelude to the blazing orange lava of popular imagination. 

Indonesia, the world's most active volcanic region, lies on the Pacific "Ring of Fire" where tectonic plates collide, causing frequent volcanic and seismic activities. 

Last year, seven people were killed after Mt. Sinabung on the western island of Sumatra erupted. A 2014 eruption at Sinabung killed 16.