Vehicles laden with food, facemasks and bedding have been sent to help more than 75,000 people who have fled a volcano on the tourist island of Bali, as the Indonesian president flew in to visit crowded aid centres.

Mount Agung, 75 kilometres (47 miles) from the resort hub of Kuta, has been rumbling since August and threatening to erupt for the first time since 1963 -- a potential blow to the country's lucrative tourism industry.

Increasingly frequent tremors show that the molten magma is still rising towards the surface, with the mountain entering a "critical phase", said the national disaster mitigation agency.

It said the number fleeing their homes had increased as fears grow that the mountain could blow.

"The local mitigation agency reported that until 12 pm Tuesday, the number has reached 75,673 people, spread across 377 evacuation centres in nine districts," said agency spokesman Sutopo Purwo Nugroho.

Around 62,000 people lived in the danger zone before the evacuations, according to the agency, but residents just outside the area have also left as a precaution.

"The number is expected to continue to rise," Nugroho said.

The Indonesian Center for Volcanology and Geological Hazard Mitigation said there has been an increase in volcanic tremors, with a total of 564 recorded Monday.

Evacuees have packed into temporary shelters or moved in with relatives. Some 2,000 cows have also been evacuated from the flanks of the volcano.

President Joko Widodo was due to visit crammed evacuation centres in Bali on Tuesday afternoon.

Balinese residents, international NGOs and the central government have begun organising aid.

Vehicles loaded with noodles, mineral water and blankets have been sent to the evacuation centres, while residents around the island have been collecting donations.

Bali's "sister village" programme and tradition of communal assistance means evacuees have been able to stay in villages outside the danger zone.

I Ketut Subandi, head of logistics at the village of Tana Ampo, said basic food items like rice, instant noodles, cooking oil and water were most needed.

"This morning we were worried because we had limited rice supply, but now we have received more rice stocks from donors," Subandi said.

Indonesia's national disaster agency has sent 640,000 face masks, 12,500 mattresses, 8,400 blankets and 50 tents. The central government has a relief fund totalling nearly $150 million to meet the cost of natural disasters, which could be tapped in case of an eruption.

Officials announced the highest possible alert level on Friday due to the increasing volcanic activity and warned people to stay at least nine kilometres away from the crater.

Operators have cancelled trekking tours on the mountain  but officials have otherwise been at pains to assure tourists the island is safe.

The airport in Bali's capital Denpasar, through which millions of foreign tourists pass every year, has not been affected, but several countries including Australia and Singapore have issued  a travel advisory.

Flights to and from the island have not been interrupted but airlines are watching the situation closely.

Virgin Australia said it would be making an extra fuel stop in Darwin for some of its flights between Australia and Bali in case it is forced to turn back.

Singapore Airlines said customers travelling between September 23 and October 2 could rebook flights or ask for a refund.

Mount Agung is one of more than 120 active volcanoes extending the length of Indonesia, which straddles the Pacific Ring of Fire.

It last erupted in 1963, killing nearly 1,600 people and sending ash as far as the capital Jakarta.