Tsunami warnings to the local population of quake-hit Sulawesi island failed on the "last mile", causing many to be surprised by waves as high as six metres, according to a German research centre that developed a warning system used by Indonesia.

Questions have arisen over why warning systems appeared to fail after a 7.5 magnitude earthquake off the coast of Indonesia on Friday.

The confirmed death toll from the quake and subsequent tsunami has reached 844 and is expected to rise further.

"The problem was the communication between local authorities and people, for example on the beach, such as in Sulawesi," Joern Lauterjung, Director Geoservices at GFZ, told Reuters TV.

Germany provided a warning system developed by GFZ to Indonesia after a devastating tsunami killed 226,000 people in 2004.

Mr Lauterjung said that system worked as planned, predicting waves up to three metres northwest of Sulawesi.

"If you look at the entire warning chain from the creation of a warning signal up to the last mile, as we call it, up to the local population in danger, there was a problem there," he said.

"For example, it appears sirens did not work and there were no warnings via loudspeaker vans from police to the local population," he added.

Authorities have scrambled to get help into quake-hit areas as survivors streamed away from their ruined homes.

Accounts of devastation have filtered out of remote areas, including the death of 34 children at a Christian camp.

Dozens of people were reported to be trapped in the rubble of several hotels and a shopping centre in the small city of Palu, 1,500km northeast of Jakarta.

Hundreds more were feared buried in landslides that engulfed villages.

Of particular concern is Donggala, a region of 300,000 people north of Palu and close to the epicentre of the quake, and two other districts, where communication had been cut off.

The four districts have a combined population of about 1.4 million.

One woman was recovered alive from ruins overnight in the Palu neighbourhood of Balaroa, where about 1,700 houses were swallowed up when the earthquake caused soil to liquefy, the national rescue agency said.

"We don't know how many victims could be buried there, it's estimated hundreds," said Sutopo Purwo Nugroho, spokesman for the National Disaster Mitigation Agency.

All but 23 of the confirmed deaths were in Palu, a city of about 380,000 people, where workers were preparing a mass grave to bury the dead as soon as they were identified.

Nearly three days after the quake, the extent of the disaster was not known with authorities bracing for the toll to climb - perhaps into the thousands - as connections with remote areas up and down the coast are restored.

Aid workers who have reached Donggala district have said hundreds of people facing a lack of food and medicine were trying to get out, but evacuation teams had yet to arrive and roads were blocked. 

Sulawesi is one of the earthquake-prone archipelago nation's five main islands and sits astride fault lines. Numerous aftershocks have rattled the region.

A witness said queues at petrol stations on the approaches to Palu stretched for kilometres.

Convoys carrying food, water and fuel awaited police escorts to prevent pilfering before heading towards the city while residents streamed out.

The state energy company said it was airlifting in 4,000 litres of fuel, while Indonesia's logistics agency said it would send hundreds of tonnes of rice.

The government has played down worries about looting though witnesses have seen incidents.

Officials say 2,800 troops have been deployed and plans were in place to send in a further 2,000 police.

Nearly 60,000 people have been displaced.