More strong quakes off WA's north coast

People in Western Australia's north have been warned to brace for more aftershocks from the nation's equal-biggest earthquake, which fortunately struck 210 kilometres offshore.

After a magnitude 6.6 undersea quake was recorded between Port Hedland and Broome on Sunday afternoon, a cluster of more than 30 significant aftershocks were detected in the same area on Monday morning.

While Sunday's quake caused those nearest on land to fear a tsunami, and was felt from Perth to Darwin, no major damage was reported.

Geoscience Australia data shows its severity matched a quake in Tennant Creek in the Northern Territory in 1988.

The nation's 10 biggest quakes have all been in WA and the NT, but the 5.6 magnitude Newcastle quake in NSW in 1989 was the most devastating, killing 13 people, injuring many others and causing billions of dollars of damage.

The Newcastle quake's epicentre was in suburban Boolaroo, 19km from the city centre, while Sunday's occurred well off the coast.

"We were fortunate it happened some 200km out to sea," WA's Emergency Services commissioner Darren Klemm told reporters.

The quake jolted items off shop shelves and caused minor damage including cracks in some buildings, probably due to cyclone-rated construction in the region.

"It certainly could be worse," Mr Klemm said.

WA Emergency Services Minister Fran Logan said locals had been spooked by the possibility of it triggering a tsunami.

Beachgoers nervously watched the waves and people in the remote Aboriginal community of Bidyadanga evacuated to higher ground, but the Bureau of Meteorology said there was no risk.

The strongest aftershock was magnitude 5.5, and Geoscience Australia seismologist Spiro Spiliopoulos warned more were likely, but there was only a small chance a larger quake would follow.

"The Tennant Creek aftershocks went for a very long time - we got small earthquakes there years after the main shock," he told AAP.

"It's not unusual. They can go on for days, weeks."

The quakes could be related to mapped fault lines in the area, Mr Spiliopoulos said.

Derby resident Jody Gaunt was having a beer with friends when they felt the quake.

"We were sitting outside and our chairs were rocking," she told AAP.

"We were thinking, 'Is this real or not real?'"

In Port Hedland, Walkabout Hotel employee Shelley, who did not wish to give her surname, said it felt like being on "one of those vibrating machines".

"It just sort of shook the building," she told AAP.

About 15 earthquakes of magnitude 6.0 and above have been detected in Australia since records began about 1901.

"We seem to get them roughly every eight years on average," Mr Spiliopoulos said.

"In 1929, there was a magnitude 6.0 and above close to where this one was, the same sort of area off the North West Shelf."