A major 7.5-magnitude earthquake struck off Papua New Guinea on Monday, official monitors said, and a tsunami warning was issued.

The epicentre of the quake, first reported as 7.7-magnitude, was near the town of Rabaul in Papua New Guinea, 787 kilometres north-east of the capital, Port Moresby, the US Geological Survey (USGS) said. Its depth was revised from 65 kilometres to 40 kilometres.

Edward Leask, a duty seismologist at Geoscience Australia, said the main earthquake struck at 10.48am Sydney time, and there was no tsunami threat to Australia.

He said the undersea earthquake was "quite strong" and was believed to be one of the most powerful recorded in the world this year.