The quake hit at a depth of 58 kilometres at 8.31 pm (1031 GMT), 65 kilometres west of Panguna town on Papua New Guinea's Bougainville Island, and 642 kilometres west of Honiara in the Solomon Islands.
The Pacific Tsunami Warning Centre said there was "no destructive widespread tsunami threat" based on historical data, but seismologists warned there could be localised wave activity and damage.
"The whole island (of Bougainville) would have been shaking quite strongly," Geoscience Australia seismologist David Jepsen told AFP.
"There's a possibility for some damage but they experience these quite often so it's likely their structures have been built to withstand them," he added.
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"I think (any damage) wouldn't be too significant."
Because the quake occurred along a plate boundary and was relatively shallow he said there was always the chance of a localised tsunami, but a full picture was unlikely to emerge until the morning due to the area's remoteness and communications delays.
The Global Disaster Alert and Coordination System said maximum 0.2-metre wave height was expected around midnight local time at the coastal village of Koiaris on Bougainville island, predicting a "low humanitarian impact" due to the magnitude and sparse population of the area.
In February the remote town of Lata in the Solomons was hit by a devastating tsunami after an 8.0-magnitude earthquake. The tsunami left at least 10 people dead, destroyed hundreds of homes and left thousands of people homeless.
In 2007 a tsunami following an 8.0-magnitude earthquake killed at least 52 people in the Solomons and left thousands homeless.