A powerful and shallow earthquake has struck off Indonesia's Sumatra island, sending panicked residents rushing from their homes in a region hit hard by quakes and tsunamis in the past.

Key points: Magnitude 7.8 earthquake strikes off Sumatra

Magnitude 7.8 earthquake strikes off Sumatra Tsunami warning cancelled, no reports of severe damage

Tsunami warning cancelled, no reports of severe damage Warnings for Australia's west coast cancelled

Indonesian authorities called off their tsunami alert within hours of the tremor, though it was still unclear if the quake had destroyed any buildings or left any casualties.

The US Geological Survey said the magnitude 7.8 earthquake's epicentre was 808 kilometres south-west of Padang, West Sumatra, and 24 kilometres deep.

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The West Sumatra Governor said a tsunami was unlikely, though National Disaster Management Agency chief Bambang Sulistyo said he still could not contact its office in the Mentawai Islands.

Disaster agency spokesman Sutopo Purwo Nugroho said tsunami warning sirens were activated and "people responded to the early warning by evacuating to higher places", but based on their analysis, "the potential for a tsunami is low".

Indonesia's search and rescue agency said no casualties had been confirmed.

"Up until now, there is no information about deaths," Heronimus Guru, the agency's deputy head of operations, said.

The tremor was felt strongly in Padang for a few seconds, residents of the city said.

People ran out of their homes to higher ground.

"There are likely to be no affected structures in this region," the USGS said on its website.

Andi Eka Sakya, an official of the National Meteorological Agency, told TVOne: "So far there have been no reports [of damage] yet."

"In Bengkulu [on south-west coast of Sumatra] they didn't feel it at all," he said.

Australian warnings stood down

The USGS originally put the magnitude at 8.2 then 8.1, before lowering it to 7.8.

Australia cancelled earlier tsunami watch warning for parts of Western Australia, Christmas Island and Cocos Island.

The magnitude 9.15 earthquake opened a fault line deep beneath the ocean on December 26 of that year, triggering a wave as high as 17.4 meters that crashed ashore in more than a dozen countries, wiping some communities off the map in seconds.

The disaster killed 126,741 people in Aceh alone.

A map shows where a shallow magnitude 7.8 earthquake struck off the west coast of Indonesia, near Sumatra. ( Supplied: US Geological Survey )

Reuters/ AFP