Some hard-hit areas remain without electricity and can not be reached due to destroyed roads, further impeding rescue efforts for those trapped in the rubble.
"The city of Palu has been devastated and first reports out of Donggala indicate that it has also been hit extremely hard".
They were told to prepare for 1,300 victims.
People are "very afraid" to return to their homes because they fear aftershocks, Siregar said.
Makassar is over 700 kilometres (435 miles) from Palu and Donggala.
Desperate rescue operations have continued across Indonesia as the level of devastation caused by last week's natural disaster continues to worsen.
Willem Rampangilei, the chief of Indonesia's National Disaster Mitigation Agency, said the grave was dug in Palu, where the majority of the casualties have been recorded and is 33 feet by 330 feet.
"There are estimated to be many victims in this area".More news: Play Streaming Games Through Chrome Thanks to Google's Project Stream
Villagers who pulled out loved ones-alive and dead-over the weekend expressed frustration that it took rescue teams until Monday to reach Petobo.
The confirmed toll of 832 dead looked certain to rise as rescuers slowly reached devastated outlying communities hit on Friday by the 7.5 magnitude natural disaster and subsequent tsunami waves as high as six metres (20 feet).
Most of the dead were from the city of Palu, but much is still unknown about other coastal towns that have yet to be properly assessed due to impassable roads, downed power lines and phone outages.
"We suspect there are still some survivors trapped inside", the head of on rescue team, Agus Haryono, told Reuters at the collapsed seven-story Hotel Roa Roa. A mass burial was being held.
The mass prison break happened after the powerful tremor sparked a wall of water that devastated the small city of Palu Friday.
This is the dramatic moment that fishermen were forced to run for their lives after a tsunami hit the island of Sulawesi in Indonesia.
On Monday, Indonesia's energy agency said it hopes to restore the electricity network within three days.
"We want these activities to have as little disruption as possible", Jasmin said.More news: North and South Korea begin removing landmines along border
Desperation exploded into anger in Donggala, the town closest to the epicentre of the massive quake and tsunami, with residents begging Indonesia's president to help them as hungry survivors crawled into stores and grabbed boxes of food. A quake in 2004 triggered a tsunami across the Indian Ocean that killed 226,000 people in 13 countries, including more than 120,000 in Indonesia. Mechanical earth-movers waited to push soil on top of the bodies. More burials were expected to follow.
The confirmed death toll of 844 released by Nugroho on Monday afternoon was an increase of only 12 since the previous day, with almost the entire total from Palu.
Rescue workers carry an natural disaster survivor, a food vendor, from the rubble of a collapsed restaurant in Palu on Sunday.
Indonesian authorities are scrambling to get food, aid and equipment onto the quake-hit island four days after disaster struck.
Thomas Lembong, chair of Indonesia's Investment Coordinating Board, tweeted Monday morning that President Joko "Jokowi" Widodo made the authorization. Authorities have prioritized the immediate burial of the dead to prevent any disease outbreak caused by decomposing bodies.
The quake and tsunami knocked out power to the island and left many residents without access to clean water and medical supplies.
Emergency workers said that the official count of 844 dead, nearly all of them from the town of Palu, was likely to rise into the thousands as rescuers reached towns and villages cut off by damaged roads.More news: Brett Kavanaugh: White Home denies limiting Federal Bureau of Investigation inquiry