Tsunami watch in effect following Indonesia quakes

A tsunami watch is still in effect for the Indian Ocean following an aftershock off the coast of Indonesia.

An 8.7 magnitude quake which was 33km deep and centred near Sumatra struck at 8:39pm NZ time, very near to the 2004 Boxing Day quake which triggered a deadly tsunami.

A magnitude 8.3 aftershock also struck at 10:43pm NZ time the United States Geological Survey reports.

Aid agency Oxfam is reporting Phuket and Krabi are being evacuated, as the sea tide is receding.

Thailand's Phuket International Airport has also been closed because of the threat of a tsunami.

A tsunami watch is in effect for the entire Indian Ocean with Indonesia, India, Sri Lanka, Thailand and Malaysia are among the countries told to be on alert.

The warning does not extend to New Zealand at this stage.

CNN's Kathy Quiano says there's panic among Indonesia residents who've been through this before.

"The people know what to do and they're now trying to move to safer ground," she says.

"So are search and rescue teams so they're not trying to make the situation worse by trying to mobilise too many people as well."

Kathy Quiano says the general message is for people to get to safe ground as soon as possible.

Seismologist Gary Gibson says initial indications show the quake was predominantly of horizontal motion, which minimises the chance of a destructive tsunami.

"I would be surprised if the shore tsunami exceeded a metre, of course it would be nothing like as bad as the 2004 one."

Gibson says the quake which triggered the Boxing Day tsunami over seven years ago had vertical shaking of the sea floor, which is the worst case scenario.

Map: Pacific Tsunami Warning Center