More than 400 flights have been cancelled with around 59,000 people affected after Bali's main airport was closed amid warnings of a potential volcanic eruption that could happen at any time.

Key points: Mt Agung volcano alert raised to highest level

Mt Agung volcano alert raised to highest level Volcano shifted from steam-based eruptions to magmatic eruptions

Volcano shifted from steam-based eruptions to magmatic eruptions All flights in and out of Bali cancelled, thousands of passengers disrupted

Indonesian authorities have raised the Mt Agung volcano alert to the highest level and have ordered people within 10 kilometres to evacuate, as the country's disaster management agency warned "a potential eruption could happen anytime".

Mt Agung has been hurling ash thousands of metres into the atmosphere, which forced the small international airport on the island of Lombok to close on Sunday as the plumes drifted east.

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All flights in and out of Bali have been cancelled and passengers on the tarmac have been disembarked, after the 6-kilometre-high volcanic ash cloud that had been blowing towards Lombok shifted towards Denpasar.

Geological agency head Kasbani said the alert level was raised at 6:00am (local time) because the volcano has shifted from steam-based eruptions to magmatic eruptions.

Video footage shows lava or "lahar", flowing from the erupting volcano and forming a river of wet cement.

"We don't expect a big eruption but we have to stay alert and anticipate," he said.

Bali's airport will have been closed for at least 18 hours when the airport's status is reassessed at 7:00am (local time) on Tuesday.

Jetstar, Qantas and Virgin have cancelled all flights in and out of Bali due to worsening conditions, saying the safety of passengers was paramount.

A Perth Airport spokesman said all flights to Bali had been cancelled until further notice, adding that passengers who had boarded a Garuda flight that had been due to depart at 7:50am (WST) were unable to depart.

"We will try to make them as comfortable as possible, it's unfortunate when these things happen," he said.

"We are advising passengers booked to go to Bali to contact their airline."

Airlines have been rebooking passengers on flights later in the week but some people have been told the earliest available flights are on Sunday.

One passenger told the ABC she had been offered a flight in mid-December.

Customers vent on social media

Stranded customers took to social media to air their frustration.

Sorry, this video has expired Airlines resumed flights to Bali Mount Agung volcano erupted for the second time in less than a week (Image: Reuters/Antara Foto Agency)

Bethanie May Gerrie wrote on Jetstar's Facebook page: "We weren't contacted by you as promised and when we did get in contact the best you could do is put us on a flight three days later.

"The volcano is likely to erupt again and we really do not want a repeat of our experience with you on Saturday and don't want to be pushed back another three days.

"I was supposed to start a new job today but instead I am here in Bali wondering when I will be able to get home."

Newlyweds Nick and Emma, from the Perth suburb of Mullaloo, had been due to fly to Bali on Jetstar this morning for their honeymoon.

When their flight was cancelled, they tried to rebook on another airline, but that was also cancelled.

Emma said the couple would head home to wait for further information from the airlines.

"[We are] pretty disappointed — this is our honeymoon, it's a slow start to a honeymoon," she said.

Emma and Nick were due to fly from Perth to Bali today for their honeymoon. ( ABC News: Graeme Powell )

Ellie Burszt McGowan was concerned about a group of schoolies trying to get to Bali.

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"They've spent 42hrs either on a plane which turned around an 1hr out of Bali on Saturday night, sitting on tarmac in Darwin, and a day at Cairns airport yesterday," she wrote on Jetstar's page.

"They flew to Melbourne last night where they're getting some well-deserved sleep. Hopefully they'll get to fly out to Bali tonight to meet up with friends and enjoy their schoolies celebrations and put some very worried parents at ease."

Jessica Watson, who has been stranded in Bali since her flight home to Sydney was cancelled on Saturday, said on Facebook she was unhappy with the way Jetstar had handled the situation.

"There had been no attempt to contact us to re-book/allocate us seats which you say would have been done from your end as per your communications on the website," she said.

"I get you are a budget airline but the communication regarding this has been terrible, after our experience at check in and the airport Saturday evening this has been handled very poorly and I will be referring this matter to the ombudsman when I return home."

ABC/AP