The Agung Volcano Observatory updated its aviation colour code from yellow to orange on Tuesday at 6.45pm local time.

"Based on our visual and instrumental observation, Agung volcano is exhibiting heightened unrest with increased likelihood of eruption," it said in a statement. Ash cloud was not visible.

Flights have not been affected at Bali's international airport, which is more than 70 kilometres from the mountain.

As fears escalated of an eruption, Indonesia's National Disaster Management Agency installed sirens to provide early warnings of an eruption.

Bali was also spooked by a 4.2 magnitude earthquake at 4.30pm local time on Tuesday, the jolt of which could be felt all the way to Denpasar.

The earthquake - only five kilometres deep - is the strongest since the volcano was upgraded to the highest warning level on Friday night.

Constant tremors indicate that molten rock known as magma is rising to the top of the mountain and it could blow at any time.

"Plume is observed emitting continuously from the main crater and reached an altitude of approximately 500 metres and moving north-west," the Agung Volcano Observatory said.

More than 75,000 people have now fled the area around the mountain in eastern Bali, which is about 72 kilometres from the tourism playground of Kuta.

Among those evacuated were 190 prisoners from Karangasem jail including Australians Bali nine drug mule Scott Rush and former Sydney kickboxer Michael Sacatides.

The head of bali corrections Surung Pasaribu said the prisoners were moved to other jails in Bali.

Most of them were trasferred to Bangli narcotics jail.

"It is for humanity and safety reason," Mr Pasaribu said. "To prevent chaos, prevention is always better then anything. I have also temporarily moved the prison staff to Bangli narcotic prison, again, for humanity reasons."

President Joko Widodo visited evacuation centres on Tuesday, including the Swecapura sports centre in Klungkung, where he handed out notebooks to children.

He said managing a volcano eruption disaster was not easy because there was no certainty when it would erupt and it was impossible to accurately predict the timing and intensity.

President Jokowi, as he is popularly known, said his government would work hard to minimise the economic loss caused because people had to evacuate but said safety was the most important priority.

He asked everyone to obey all instructions from local authorities to minimise the impact of Mount Agung.

"And we all pray to God and Sang Hyang Widhi Wasa (supreme Hindu God) to lessen this ordeal."

Meanwhile, the director and founder of the Ubud Writers Festival, Janet DeNeefe, said it was too early to speculate whether an eruption would impact on the international festival, scheduled for October 25-29.

"We will update all ticket holders of any changes to the event as we move closer to the date but for now - still one month out from the event - it's on with the show."

Donations are flowing, with nappies, formula, blankets, healthy food - and a spot for cows - among the highest priorities.

Ade Andreawan from IDEP, an Indonesian NGO, urged the public to donate food that was healthier than instant noodles, saying eggs, vegetables, tea and coffee would be more useful.

"Who knows if people will be here for a month, two months ..." he said.

IDEP was providing activities for children stranded at the centre and teaching them to cover their noses, eyes, mouths and body if the air was thick with volcanic ash in the event of an eruption.

The International Volcanic Health Hazard Network warns freshly fallen volcanic ash particles can have acid coatings which may cause irritation to the lungs and eyes.

Mr Andreawan said Denpasar had run out of the face masks recommended for volcanic ash and IDEP was trying to source more from Jakarta and Surabaya.

Indonesian Disaster Management Agency spokesman Sutopo Purwo Nugroho said masks were available at the agency's posts but had not yet been distributed.

"It's not urgent unless Mount Agung erupts - then the masks will be distributed," he told Fairfax Media. "What we are distributing now is food and daily needs."

Ngurah Rai Airport spokesman Arie Ahsan said if volcanic ash was present the airport would also distribute masks.

He said the masks would be in accordance with health department standards for volcanic ash.