Rumbling Mount Merapi has grown 10 metres in height in three days with pools of lava forming at its smoking summit.
Experts fear an eruption could be imminent.
Villagers living on the slopes of the 2,914-metre peak has been on standby alert for more than three weeks.
That is one level below that which would require a mandatory evacuation for more than 29,000 people living around its fertile slopes.
Volcano watchers said a new dome of lava had formed in recent days and was getting bigger all the time, while yellow clouds of sulfur continued to spew out.
"The dome has appeared on the southern part of the peak, while tremors are fluctuating in high numbers," volcanologist Subandriyo said.
Lava domes meant the cone-shaped volcano could erupt any day and its top has grown as trapped magma from a reservoir 1.5km below the summit began welling up on Sunday, he said.
The collapse of a lava dome can cause superheated pyroclastic flows of gas down the mountain sides.
During the last eruption in 1994 hot gas clouds - called "shaggy goats" by locals - travelled at fast speed several kilometres down from the summit and killed 43 people, mostly from horrific burns.
The force of that eruption flattened houses and crops and killed livestock.
In 1930 at least 1,300 were killed in an eruption.
Authorities have yet to order villagers living on the mountain to evacuate this time.
However, after the revered local ruler, Sultan Hamengkubuwono X of Yogyakarta, has pleaded with people go, some pregnant women and children left voluntarily for refugee shelters in the villages of Argomulyo, Wukirsari and Gayam.
Yogyakarta, 20 kilometres from the mountain, is a centre for several leading Indonesian universities.
Many Australian students study there.
Travel warnings from Canberra say Australians staying or living in the area should be prepared to evacuate along routes already prepared by local authorities.
Subandriyo," the head of volcanology of the Yogyakarta Agency for the Assessment and Application of Technology, said Merapi was ready to erupt at any time.
"We no longer need to do measurements. Magma is already at the surface," he told Indonesian media.
Many local people have been reluctant to leave the mountain, preferring to take the advice of local mystics who believe Merapi will only erupt after certain omens including mysterious beams of light shining over its steep flanks.
Some have slaughtered goats and chickens as offerings to the mountain, while others have held joint Muslim and Christian prayer gatherings.
The mountain has spiritual significance for many Javanese and is one of only four places where the royal palaces of Yogyakarta and Solo make offerings to placate the ancient Javanese spirits.
Merapi sits atop the unstable meeting point of the Australian and Asian geographic plates, creating the so called "ring of fire".
A large eruption in 1006 covered all of central Java with ash, destroying the local Hindu Kingdom and opening the island to Muslim rule. http://au.news.yahoo.com/060414/2/yln4.html