Indonesia's tsunami early warning system, widely praised during this week's Indian Ocean-wide alert, has been compromised by thieves and vandals in the country's waters, officials said Saturday.

The $130-million system of tidal gauges, buoys and seismic monitors sent warnings to Indonesian authorities Wednesday after an 8.6-magnitude quake struck off Sumatra island.

Ten people died in the earthquake, mostly elderly people suffering heart attacks from shock, and little damage was reported to buildings and infrastructure.

"We have had problems with theft and vandalism of our system for a while," Indonesian National Disaster Mitigation Agency spokesman Sutopo Purwo Nugroho told AFP.

"We got the tsunami warning from a seismograph but because so many of the buoys are destroyed we can't tell how big a potential tsunami would be."

Nugroho said just three of 25 buoys in Indonesian waters were in operation, mostly because of vandalism.

Since the system was set up in 2008, fishermen have reportedly used the buoys to moor their vessels, at times damaging the instruments.

Indonesian authorities are now working with the United States to try and develop deeper-water equipment to prevent theft and vandalism, officials said.

Experts told AFP the system had functioned well in Aceh province on the northern tip of Sumatra, where 170,000 people were killed and entire towns were flattened by the massive Indian Ocean tsunami in 2004.

They said government agencies sent SMS messages and sounded sirens to warn people to search for higher ground.

But an AFP correspondent in Aceh province said some sirens did not work and the government did not sound the sirens until 45 minutes after the quake.

Most people began running uphill as soon as they could, many remembering the 2004 tsunami had thrashed Aceh within 15 minutes of the earthquake.

"Only three of the six sirens in Aceh worked because someone at one of the receiver stations panicked and evacuated the building without activating the sirens," Nugroho said.

"Others were poorly maintained, and weren't loud enough."

After an oceanwide alert was issued Wednesday, countries such as Thailand, India and Sri Lanka helped spread the word through SMS messages, smartphone apps and social media.

But the time lag and poor maintenance of equipment in Aceh has raised concerns that Indonesians will be unprepared should a tsunami hit.

Geologists have predicted an 8.9-magnitude will in coming years hit the Mentawai Islands of Western Sumatra, where there are no sirens.

Damage in Sumatra was minimal because the epicentre was much further offshore than in 2004, according to the Meteorology and Geophysics Agency.

Government regulations have also ensured buildings are built with better resistance to quakes, according to the United Nations Development Programme, and people were better prepared.