DENPASAR, Indonesia -- The international airport on the Indonesian resort island of Bali reopened Friday afternoon after a nearly 12-hour closure due to a volcanic ash threat that disrupted travel plans for thousands.

Nearly 450 flights were cancelled Friday, affecting some 75,000 people, as the Mount Agung volcano gushed a 2,500-metre column of ash and smoke for a second day.

The National Disaster Mitigation Agency said tests showed there was no ash in Ngurah Rai International Airport's airspace and the airport reopened at 2.30 p.m.

Airlines are likely to remain wary, however. Australian airlines had cancelled flights scheduled for Thursday evening while the airport was still operating. The airport's online flight schedule showed Singapore Airlines and KLM flights scheduled to arrive Friday evening.

Australia's national airline Qantas said it was monitoring advice from the regional Volcanic Ash Advisory Center in Darwin, Australia, and its own pilots and meteorologists would decide when flights can resume.

Volcanic ash is a potentially deadly threat to aircraft that can cause engines to "flame out."

"We hadn't a place to stay for the night so we had to find something else, just took a taxi and stayed at a random hostel," said a stranded German backpacker who identified herself as Louisa.

Two small airports, at Banyuwangi and Jember in eastern Java, also closed because of the ash threat.

Agung's alert level has not been raised and an exclusion zone around the crater remains at 4 kilometres.

The volcano, about 70 kilometres northeast of Bali's tourist hotspot of Kuta, last had a major eruption in 1963, killing about 1,100 people.

It had a dramatic increase in activity last year, forcing the evacuation of tens of thousands of people, but had quietened by early this year. Authorities lowered its alert status from the highest level in February.

Indonesia, an archipelago of more than 250 million people, sits on the Pacific "Ring of Fire" and is prone to earthquakes and volcanic eruptions. Government seismologists monitor more than 120 active volcanoes.