"If a tsunami has been generated it is not likely to arrive in New Zealand for at least 4 hours", it said in a tweet. It was centred at a relatively shallow depth of 10 kilometres, it said.
MCDEM initially said it was assessing the potential tsunami risk to New Zealand.
A strong quake in Papua New Guinea triggers a tsunami warning there and for the Solomon Islands. It said there was no tsunami threat to Hawaii or Guam.
Scientists do not have sea level gauges in this remote corner on the Bismark Archipelago, but they later said they believed the threat had passed.
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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.