South Lampung is a region of Sumatra, a separate island, which is about 190 km north west from Jakarta.

At a small government-run medical clinic by the beach in Anyer in Banten, one of many along the road through the affected areas, survivors gathered to get medical care.

At just one small site, 12 people - 6 men, 3 women and 3 children - were killed.

Another 33 people were injured and 36 were missing.

Rahmat Riyono, the head of Meteorology, Climatology and Geophysics Agency (BMKG) said at a press conference on Sunday morning that - based on media reports - the tsunami was about two metres high.

But this was just an estimate, he said, and the agency now had experts in the field evaluating the aftermath.

Some news organisations described people running in fear for their lives from waves up to 5 metres high.

He also flagged that another tsunami was possible because Saturday night's wave had been caused by an eruption of the Anak-Krakatau volcano, rather than an earthquake.

"We are waiting for a status update from Anak-Krakatau. The chance of a tsunami returning a second time is very small if it caused by an earthquake. But since this is caused by an eruption, it is a different case. We have to continue monitor."

"There was no early warning, because it's a tsunami caused by a volcano."

"We urge people to stay away from Anak-Krakatau and the beach."

A machine that monitors volcanic activity on Anak-Krakatoa, which is 156 km west of Jakarta, was damaged by the volcano’s eruption, the BMKG said.

This meant that authorities did not have enough accurate data about the event.

In turn, there was confusion initially over whether a tidal wave - rather than a tsunami - had been triggered by the eruption.

It also meant the tsunami early warning system was not triggered.

Among those to have perished in the natural disaster were the band manager and bassist for the Jakarta pop band Seventeen, who were performing at resort in Tanjung Lesung when a wave collapsed the stage from behind.

In chilling footage posted online, the band are seen performing on stage before the wave strikes and sweeps them into the audience.

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The concert was for around 300 employees of the state-owned electricity company and was at the Tanjung Lesung beach, in Banten province.

In a statement, the band confirmed that its bass player, M. Awal Purbani, and road manager Oki Wijaya had lost their lives.

Guitarist Herman Sikumbang, drummer Andi Windu Darmawan, crew member Ujang and Dylan Sahara, the wife of lead singer Ifan are still missing too.

Editor-in-chief of iNews Media Masirom was holidaying in Karangbolong Beach at the time the tsunami hit.

"Around 9.30pm I was relaxing inside the bungalow, suddenly there was a loud noise, like an explosion ... the playground where my children played earlier has been filled with sea water," he said.

"The panic due to this disaster has not completely subsided. Moreover, it is now raining heavily. Our family [is] not just fighting off feelings of worry, but also the cold weather."

Oystein Lund Andersen, an employee of the Norwegian embassy in Jakarta, was also on holidays in Anyer, on the Javanese coast with his family when the wave hit.

"I was myself at the beach photographing the well known volcano - Anak Krakatau, when I suddenly saw a big wave," he wrote in a Facebook post.

"I had to run, as the wave passed the beach and landed 15-20m inland. [The] next wave entered the hotel area where I was staying and downed cars on the road behind it."

Indonesian president Joko Widodo said in a statement that he had instructed the army, disaster agencies and the social affairs ministry to work together to handle the disaster.

"I'd like to extend my deep condolences to the [families of] victims in Banten province, Serang and Pandeglang. I hope the families will have strengh [coping with this incident],” he said.

The pre-Christmas tsunami is the latest in what has been a series of natural disasters that have struck Indonesia in 2018, including earthquakes that hit the resort island of Lombok in August and an earthquake and tsunami that struck the city of Palu and the surrounding region, on the island of Sulawesi, in late September.

Endan Permana, an official from the disaster agency BNPB's local office in Pandeglang said he hoped that not more than 100 people had been killed within the region.

Four districts in Pandeglang had been affected, he said: Sumur, where 12 people found were found dead on Sunday morning, Panimbang, Carita and Labuhan.

Permana said about 20 locals were thought to be missing in the resort region of Tanjung Lesung, about five hours by road from Indonesia's capital of Jakarta and close to where the tsunami struck.

"We have problems in Tanjung Lesung because there has been heavy rain and we are short of personnel," he said.

"The military and police and local people are helping local victims but we still need more people to help in Tanjung Lesung."

A spokesman for the Department of Foreign Affairs and Trade said the Australian Embassy in Jakarta was "making urgent enquiries to determine whether any Australians have been affected by the tidal wave that hit beaches in the Sunda Strait area."

Prime Minister Scott Morrison desribed the "high-wave event" as a "terrible blow for Indonesia."

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"We understand that at present there are no foreigners, let alone Australians, who have been impacted by this...This comes on top of what happened in Sulawesi and so, as always, we're available to support the Indonesian government with these things, as requested."

He said there had been no requests for assistance, and he was not anticipating any on this occasion.

The 1883 eruption of Krakatoa was one of the deadliest volcanic eruptions in the world, killing more than 36,000 people.

With wires