A short time later, it said it had assessed the quake with the assistance of science advisors, and "the initial assessment is that the natural disaster is unlikely to have caused a tsunami that will affect New Zealand".
There were no reports of major damage or casualties.
"We are conducting the assessment starting this morning", he told Reuters news agency by telephone.
Police Sergeant Frank Kilaur described the tremor as "very strong".
The warning covers the Solomon Islands and Papua New Guinea. The reported epicentre lies within the so-called Pacific Ring of Fire, an area of regular seismic activity.
A 7.5-magnitude quake hit the rugged highlands region in February 2018 that triggered landslides, burying homes and killing at least 125 people.