Sydney - The northern Australian city of Darwin was rocked by an offshore earthquake on Monday but there were no reports of injuries or damage.

Geoscience Australia duty seismologist Cvetan Sinadinovski said a tremor was felt after the 5.3 magnitude quake hit 500km north-west of Darwin.

"People would have felt this earthquake up to 500km away from the epicentre, and we have heard numerous reports of residents in the Darwin area having felt the effects of the event," Dr Sinadinovski said.

The Bureau of Meteorology's Darwin office was also shaken by the earthquake.

"I felt the building sway and some of the monitors shook a little," the bureau's Billy Lynch told Australia's AAP news agency.

"We are on the third floor of a three-storey building."

Dr Sinadinovski said the biggest earthquake to strike the region, in 1998, had a magnitude of 6.9.

"This event is an inter-plate earthquake that occurs when the Australian and Asian plates collide," he said.

"This is a dynamic process which is a result of the movement of the tectonic plates which form the Earth's crust." - Sapa-dpa