The official death toll has now risen to 844 according to National Disaster Management Board spokesman Sutopo Purwo Nugroho.

Sutopo said Indonesia had 4500 to 6000 earthquakes a year but - due to limited budget - had just 60 units of tsunami sirens. "We need thousands of them," he said.

Twenty-five to 35 buoys designed to warn about a tsunami approaching Palu had been out of order since 2012, Sutopo said.

"Some people stole them. Some did vandalism on the units as they didn't know what they were."

Senior Indonesian officials have met in Jakarta with ambassadors from countries – including Australia, the US, South Korea, China and Singapore – that have offered assistance.

That meeting came after President Joko Widodo told Foreign Minister Sri Mulyani that Indonesia would accept international assistance – unlike after the Lombok earthquake seven weeks ago.

The international assistance would be "selective" and would prioritise planes that can land on 2000-metre runways, tents, water and sanitation systems, power generators, field hospitals and medical assistance.

More than 600 people have been badly injured and there are 48,025 refugees in Palu alone, a coastal city with a population of 374,000 that was flattened by the giant wave.

A mass grave for the victims is being prepared in Palu, with corpses to be buried after being identified to avoid the spread of disease.

Sutopo said proper burial procedures would be followed, with men and women divided, as had been the case in the burial after the 2004 tsunami in Aceh.

A 14-day state of emergency has been declared in Central Sulawesi, where a lack of heavy equipment and chronic shortage of fuel was making it difficult to recover victims.

Sutopo said heavy equipment was being transported from Makassar to assist with evacuation works, which were focused on a collapsed eight-storey hotel in Palu and other residential areas.

He said it was still unknown how many lives had been lost in Petobo, in South Palu, and the Balaroa residential area in nearby Sigi, which fell exactly on the earthquake fault-line.

"We predict hundreds of people are still buried under the rubble of the Balaroa residential area in Palu."

Sutopo said 744 houses in Petobo, in South Palu, had been impacted by liquefaction, a process where the sheer amount of liquid in the soil turned it into a watery mud.

Petobo resident Amirudin U Labugis told Fairfax Media his house was swallowed by a “tsunami of mud”.

“It lifted my house, seven meters up, then it took it away, it moved in circles, like washing machine,” he said. He has no idea how they survived.

“It was luck that we ran towards safety and were not gobbled up by the mud,” he said. “We took refuge up a hill. I ran without anything, no food, no water.”

Amirudin said his youngest son had been sick with vomiting, diarrhoea and fever. “We need food, water. We ran out, after this afternoon we won't have anything. No shops are open, so we can't buy anything."

About 1.6 million people are estimated to have been affected on top of those killed and injured. Seven districts were still isolated in Sigi, the south of Palu, due to landslides.

"Whilst the Australian government has not yet received a formal request, we are working closely with Indonesian counterparts to determine what assistance Australia could most effectively provide to meet the needs of those impacted by the disaster," a spokesman for Foreign Affairs Minister Marise Payne told Fairfax Media.

Associated Press reported that an early warning system that might have prevented deaths in the tsunami had been stalled in the testing phase for years.

It said a high-tech system of sea-floor sensors, data-laden sound waves and fibre-optic cable was meant to replace a system set up after the 2004 tsunami in Aceh but delays meant the system hadn't moved beyond a prototype.

Telecommunication services are being restored in Palu and surrounding areas, as Telkom Indonesia works to repair infrastructure damaged in the quake. Government buildings were being prioritised.

The tsunami that hit Palu, Donggala and Mamuju in Central Sulawesi on Friday evening was triggered by a 7.4 magnitude earthquake.

Tsunami survivor Rosina Mursidin yelled for her children to get out of the house after the earthquake.

"That's when we all saw it, it wasn't your regular wave, it was the colour of cigarette ash," she said. "So me and my family and a few neighbours, about 20 people, climbed the mango tree in the yard. Just as the water hit us, the tree fell, it pinned my mother, but we hung onto it."

Rosina said she gripped the twig from which her two-year-old nephew was hanging and prayed: "Please God save him, he is innocent".

"All the while I prayed, I was totally submerged, I drank a lot of water, barely able to breathe, but I kept my grip up. I don't know for how long, but when the water slowed and lowered down I took a deep breath and check my family.

They survived but others have not been so lucky.

Callouts for missing people are being posted on Twitter.

Puji Lestari posted she was looking for her 20-year-old sister, Denny Ayu Lestari, whom she had been unable to contact for a day.

“A day after the quake, she called my mom twice because mom called her every second. She said she was alright, she was up in the mountain with her friends,” Puji told Fairfax Media.

“She told my mum that she needs food and clothing.”

Puji said her sister’s mobile was completely off and her friend’s phone was also inactive because the network was still in trouble.

Oxfam is planning a response to reach 100,000 people in Palu city and Donggala district. This is likely to focus on the immediate needs such as ready-to-eat meals, water purification kits and emergency shelters," said Ancilla Bere, Oxfam’s Humanitarian Manager in Indonesia.

Care Australia has also launched an appeal and is calling for urgent donations to help it deliver clean water and emergency food supplies.

To donate, visit Red Cross Indonesia Earthquakes and Tsunami Appeal, Oxfam Australia, care.org.au/tsunami or call 1800 020 046