Idusan, 9, a Sri Lankan tsunami survivor, looks at the camera at a shelter for displaced tsunami survivors in Thambiluvil on Sri Lanka's east coast in this January 24, 2005 file photo. On December 26, 2004, a magnitude 9.15 quake off the coast of Indonesia's Aceh province triggered an Indian Ocean tsunami that killed around 226,000 people in Indonesia, Sri Lanka, India, Thailand and nine other countries. [Photo/Agencies]

BANGKOK - In April 2012, Indonesia's Banda Aceh, the city worst hit by the tsunami that killed at least 226,000 people on Boxing Day ten years ago, received a terrifying reminder of how unprepared it was for the next disaster.

As an 8.6-magnitude quake struck at sea, thousands of residents shunned purpose-built shelters and fled by car and motorcycle, clogging streets with traffic. A network of powerful warning sirens stayed silent.

No wave came. But if it had, the damage would have been "worse than 2004, if it was the same magnitude of tsunami", said Harkunti Rahayu, from Indonesia's Bandung Institute of Technology.

As the 10th anniversary of the disaster approaches, experts and officials say weaknesses remain across the region in a system designed to warn people and get them to safety.

For millions in coastal areas, warnings don't always get through, thanks to bureaucratic confusion and geography. In the most vulnerable areas, infrastructure is wanting, and many lack the basic knowledge to keep themselves safe from the deadly waves.

Since the disaster, a sophisticated early warning system has sprouted from next to nothing, costing over $400 million across 28 countries.

With 101 sea-level gauges, 148 seismometers and nine buoys, the Indian Ocean Tsunami Warning System can send alerts to countries' tsunami warning centres within 10 minutes of a quake, Tony Elliott, the head of the UNESCO secretariat that oversees the system, told Reuters.

But there has also been mismanagement and waste.

In Indonesia, a German-funded detection initiative built an expensive network of buoys - and then scrapped them - after reports of cost overruns and signs they were ineffective.

All but one of nine Indonesian-operated buoys had been lost or damaged by fishermen, said Velly Asvaliantina, an official at Indonesia's Agency for the Assessment and Application of Technology.

The remaining buoy is not operational, she said.

Elliott said technological advances mean the lack of buoys is not a significant impediment in tsunami detection.