A scientist who predicted the second Indonesian earthquake fears a third devastating jolt, powerful enough to cause another major tsunami, is "imminent".
The waves could sweep north-western Australia, reaching as far as Perth.
John McCloskey, of the University of Ulster, said building the Indian Ocean tsunami warning system was "an urgent priority".
"Don't take the foot off the gas. This is very urgent work."
In mid March, Professor McCloskey warned that the Boxing Day quake, which triggered the tsunami that killed 300,000 people, had shifted tectonic stresses to another spot on Sumatra's geological fault line.
He predicted a second strong quake, noting many did not believe lightning could strike twice. "But with earthquakes it's exactly the opposite ... I quite honestly hope we're completely wrong."
He wasn't. The second quake, measuring 8.3, struck on March 28 near the Simeulue and Nias islands, killing 2000 people.
In a new study, published in Nature, Professor McCloskey's team reports that "stresses imposed by the second rupture have brought closer to failure" another zone "immediately to the south, under the Batu and Mentawai islands".
"The historical record and the experience of the Sumatra-Andaman and Simeulue-Nias events indicate that a tsunami could be a possibility."
Professor McCloskey told the Herald it would likely strike near the Mentawai islands, triggering a repeat performance of the 8.5 quake of 1833. "The 1833 earthquake is probably a reasonable model. It did trigger a tsunami and there were many casualties. That's the type of earthquake we fear it definitely could be."
Professor McCloskey noted that the 1833 tsunami reached north-western Australia. Next time "the waves would be felt in Perth," he said, adding he could not say how strong they would be.
It was impossible to say when it would happen, but the evidence, including historical data, showed it could be within 30 years, following the pattern of the 1833 and 1861 Sumatra quakes.
"It may be sooner. We must assume it's imminent and behave accordingly. We can't bury our heads in the sand."
Commenting on his last prediction, Professor McCloskey said: "I've very mixed feelings."
He had "a sense of professional satisfaction that our science has started to understand well" earthquakes. "I hope I am wrong this time, but I don't think so. It's not something you get any pleasure out of ... even though with the last one we were very accurate."
While a "high tech" warning system would protect people around the Indian Ocean, there would be no time to alert Sumatra. A program was needed to teach them how to save themselves.
"People need to plan what to do in Sumatra when they feel the earth shake. You have 15 to 20 minutes to get yourself into a position safe from the tsunami."