The 7.5 magnitude quake struck, just six miles below the surface, at 6pm local time (11am BST) on Friday.
Indonesia’s meteorological and geophysics agency BMKG issued a tsunami warning just after the initial quake, warning of potential waves of 0.5 to three metres – but lifted the warning after just 30 minutes.
But then Palu, a town on Sulawesi, was hit by waves as high as six metres, causing sheer devastation as the wall of water devoured everything in its path.
More than 840 have been confirmed dead, and, with scores still missing, that figure is expected to rise significantly.
Many critics have accused BMKG of lifting the warning too early, though the agency says the waves hit while the warning was still in force.
A spokeswoman for BMKG said the tsunami alert ended at 18:37, minutes after the third and last wave hit land.
But there was a bigger problem: The earthquake had wiped out the area’s power and communication lines.
This means the tsunami alerts – sent out to residents via text message – may never have been received.
Another spokesman said there are also no sirens in place along the coast which could act as a catch-all alert for those without phones.
And the consequences were horrific.
Hundreds of people had gathered at the beachfront for a festival, which ended in scenes of horror.
One chilling video shared on social media shows people milling around the beachfront, while the man filming screams desperately to alert them to the massive wave approaching behind them.
While Indonesia does have a tsunami warning system, which is why the alerts were sent, the head of BMKG’s earthquake and tsunami centre told the BBC they were “very limited.”
Rahmat Triyono said: “Our [current] tools are very lacking.
“In fact, of the 170 earthquake sensors we have, we only have a maintenance budget for 70 sensors.”
The tidal gauge, which measures changes in sea level, only recorded an “insignificant” 6cm (2.5in) rise.
The tsunami height was estimated to be less than 0.5m, BMKG said, which was deeply flawed.
BMKG revealed that the nearest closest tidal gauge to Palu was one that was well over 200km away.
Mr Triyono told Reuters: “We have no observation data at Palu. So we had to use the data we had and make a call based on that.
“If we had a tide gauge or proper data in Palu, of course it would have been better. This is something we must evaluate for the future.”
This disaster, and all the lives lost, has highlighted the costs of Indonesia not having implemented a more sophisticated early warning system.