The warning was raised to the maximum level four on Friday night, which means a hazardous eruption is imminent for the first time in 54 years.

This could happen before Sunday morning.

Locals reported monkeys and snakes fleeing from the mountain.

People have also been told to evacuate from within a nine to 12-kilometre radius after smoke was detected rising 100 metres from the summit on Friday night.

Travel to Bali

The airport was still operating on Saturday afternoon but a series of tremors increasing in intensity have shaken the area surrounding Mount Agung.

As of 2.30pm local time (5.30pm AEST) there had been more than a dozen earthquakes on Saturday. The largest of 3.7-magnitude and at a depth of six kilometres.

More than 20,000 Australians expected to fly to Bali in the next two weeks could be left stranded on the island if the volcano erupts.

An increased number of families are expected to travel to the popular travel destination, with school holidays now under way in NSW, Victoria, Queensland, Western Australia and the ACT, and commencing next week for schools in South Australia, Tasmania and the Northern Territory.

A spokesman for Jetsar said the airline runs 60 direct flights to Bali from major Australian cities every week, transporting more than 10,000 passengers.

He said all services were running as scheduled, but the airline was "keeping a close watch on the situation".

The Australian Department of Foreign Affairs on Friday issued an updated travel warning for Indonesia that said an eruption could impact air travel in the region.

"Contact your airline or tour operator to confirm travel plans," it said.

It also warned Australians to monitor local media and follow the instructions of local authorities.

A Qantas spokeswoman said its meteorologists were keeping a close eye on conditions and the airline would make changes to flights if the situation escalated.

"We are closely monitoring the activity of Mount Agung, but there is no impact to our services at this stage," the spokeswoman said.

A Virgin spokeswoman said the airline was not currently experiencing delays or cancelling flights.

"At this stage, operations are continuing as normal but we will continue to monitor the situation closely," the spokeswoman said.

Mount Agung, which is 71 kilometres from the tourist destination Kuta, last erupted in 1963, killing 1100 people.

The large volcano has a peak 10,000 feet above sea level.

Hundreds of tremors

About 500 tremors were recorded in the region of Mount Agung on Friday between 6am and 6pm local time alone, with the highest measuring 3.6-magnitude. That tremor was 29 kilometres deep.

An earthquake in the precinctat about 5am Saturday local time measured 3.7 and caused locals to panic.

The Volcano Observatory Notice for Aviation said seismic activity showed a tremendous increase in numbers and activity.

"This number of seismicity is an unprecedented seismic observation at Agung volcano ever recorded by our seismic networks," it said in a statement.

It said its monitoring data and analysis indicated an increased probability of eruption but it could not estimate exactly when it would take place.

National Disaster Mitigation Agency spokesman Sutopo Purwo Nugroho announced on Friday night the warning level had been raised from level three to level four.

"With the expansion of the hazardous zone area the refugees will increase " he said.

Mr Sutopo urged people to "calm down" and not be provoked by misleading information.

"Until now Mount Agung has not erupted. Monitoring has intensified."

Centre for Volcanology and Geological Hazard Mitigation Kasbani said on Friday night that the evacuation from the affected area should begin immediately.

"The development today is quite extraordinary, the intensity of it," he said.

Villagers flee

Kadek Nadi and his large family of 22 evacuated two days ago from their village Bukit Galah - Pasar Agung, about 4km from the summit. They are now seeking refuge at a sport centre in Klungkung.

Kadek's grandfather, I Wayan Pugeng, and his family survived the 1963 eruption.

"They evacuated two days before Mount Agung erupted the last time," Kadek said.

Kadek's grandfather warned the family to keep an eye on the wild animals.

"Monkeys, birds, snakes and deers, all fled the mountain [in 1963]. My grandfather's family evacuated when they saw the animals flee. That's how they survived then."

"I didn't see any wild animals this time, we voluntarily evacuated because of the tremors. Up there, back home, it was non-stop. So we packed up and left. All of our cows are still up there, my family members are taking turns to care for them. Our cows are not panicking or anything, but they are restless, they don't eat, when they do, they eat very little."

Kadek said he would take an offer from anyone who would buy his cows. "Usually we can sell them for 10 million rupiah [$1000] each, now any offer, we would take it. Four or five million rupiah anything, it's better then nothing."

He said most of his family were vegetable and rice farmers but they raised cows on the side.

Kadek said when the family arrived at the sports centre in Klungkung there were only a few hundred evacuees and logistics were easy.

"Now I think it's more than 5000 and more coming, and we have to wait in quite a long line. Also there are only 12 toilets for everyone."

Evacuees, who are living in tents or sports centres, need assistance with nappies, sanitary napkins and baby food. There is also a huge demand for portable toilets.

Emeritus Professor Richard John Arculus from the Australian National University wrote that although infrequent, eruptions of Mount Agung have been among the largest of the past 100 years of global volcanic activity.

"Mount Agung is one of many similar volcanoes in Indonesia and the ring of fire surrounding the Pacific and eastern Indian oceans," he wrote in upi.com.

"But during its sporadic eruptions, Agung has been one of the most prominent injectors of volcanic ash and sulphur dioxide into the atmosphere."

Professor Arculus said the ability to predict eruptions had improved dramatically and it was hoped the high death toll of 1963 would not occur again.

The 1963 eruption was also preceded by earthquakes. Lava and small explosions of volcanic ash began in February that year leading to a major explosion on March 17.

There was an eruption of similar intensity in 1843 and several in the 16th to 18th centuries.

Wayan Pugeng was 12 when Mount Agung erupted in 1963. He said the family fled, leaving behind their cows, who were all killed in the eruption.

"Booom! The sound was so magnificent. There were earthquakes, then it started raining stones. There were hot clouds and then all the walls in our house collapsed," Mr Wayan said.

Back then, the family could not return to their homes for six months.

Ni Ketut Arnawi, who lives in Singaraja, about 80km from Mount Agung, describes the morning turning black when the volcano erupted in 1963.

"We didn't feel anything, no earthquake, we didn't hear anything either," she said.

"But that morning, it was just dark, then people start shouting 'It's raining sand'."

Later she learnt Mount Agung had erupted.

"It stayed dark the whole day. We didn't see the sun at all that day."

"But other than the darkness and the raining sand, our activity was normal. Maybe because we were far away."

with Joe Hinchliffe