Bali's Mount Agung volcano has reached a "critical" stage with 500 tremors shaking the area a day, according to Indonesia's National Disaster Management Agency.
Spokesman Sutopo Purwo Nugroho warned that molten rock known as magma was just three to five kilometres from the summit of the mountain in eastern Bali.
"We cannot predict when it will erupt, but monitoring shows the potential to erupt is high," Mr Sutopo said. "No instrument can predict with precision when a volcano will erupt".
He said 62,000 people had been living in the exclusion zone, but almost 50,000 evacuees had sought shelter in 301 camps in the area.
But there were still people who refused to be evacuated because the mountain had not yet erupted and they worried about who would feed their animals.
Nyoman Kastawan has remained in his village of Dukuh Pandan Sari Kubu, which is inside the exclusion zone and about eight kilometres from the summit.
He said about 40 women and children from his family had been evacuated to Denpasar, but he stayed behind to look after his cattle and pigs.
"I don't want to sell them while the price is so low," he said.
"Around 30 males still live in my village, the able-bodied, so when there is something we can evacuate immediately. Some of the old men who stayed behind said the eruption is still a long way off. They survived the 1963 eruption so they don't want to evacuate now. It's still a few months away they say."
Mr Sutopo warned that when Mount Agung last erupted in 1963, more than 1500 people were killed and 1700 houses destroyed.
"The kind of eruption is explosive, spewing stones, lava and pyroclastic flows (a fast-moving current of hot gas and volcanic material)," he said.
Indonesia's president, Joko Widodo, said he would visit refugee camps on Tuesday.
Mount Agung is 71 kilometres from the tourist hot spot of Kuta. Mr Sutopo said tourism was still normal and flights had not been affected.
He said ash and thick smoke would not reach Bali's capital, Denpasar. The travel advisory only warned people not to venture within nine kilometres south-east of Mount Agung and 12 kilometres, north, north-east, south and north-west of the crater.
Mr Sutopo said contingency plans were in place if an eruption meant flights had to be-rerouted to cities outside Bali.
He said if the volcano erupted between September 25 and October, volcanic ash would fly south-west. If it erupted between November and January the ashes would disperse to the east towards West Nusa Tenggara and East Nusa Tenggara.
The governor of Bali, I Made Mangku Pastika, has declared a state of emergency and will assist affected regions.
"Some two trillion rupiah has been allocated for the emergency situation," Mr Sutopo said.
Villages neighbouring those that had been evacuated would provide shelter under a "sister village" model.
Mr Sutopo said he deeply regretted hoaxes, which suggested the volcano had already erupted, which he believed were circulated to increase traffic to websites.
He urged the public to visit the official website for Mount Agung updates: magma.vsi.esdm.go.id.
Volcanologist Ray Cas warned the highest danger posed by an eruption would be pyroclastic flows, which were responsible for many of the deaths when Mount Agung last erupted in 1963.
He said volcanic mud flows, which channel into major valleys, could bury entire villages.
"If the population hasn't been evacuated there is a risk of a huge loss of life," Professor Cas, an Emeritus professor at Monash University and the immediate past president of the International Association for Volcanology and Chemistry of the Earth's Interior, said.
"It is not just the preliminary eruption that poses enormous hazards and risks."
He said Indonesia was one of the most volcanically active countries in the world and the Centre for Volcanology had a lot of experience in monitoring these types of events.
"They are obviously taking it extremely seriously ??? because of such a high level of earthquake activity and such a long repose period since the last eruption."
The 1963 eruption, which shot gas and ash up to 10 kilometres in the air, was ranked five on the Volcanic Explosivity Index, equivalent to the 1980 eruption of Mount St Helens in Washington, which killed more than 50 people.
In Australia, airlines are closely monitoring Bali but said on Monday afternoon that flights were planned to operate as scheduled.
Jetstar and Virgin were allowing customers to cancel or change their bookings, in return for flight vouchers.
A Jetstar spokeswoman said "a handful" of customers have requested to change their dates, which the airline had accommodated with no fee.