The Pacific Tsunami Warning Centre (PTWC) had initially warned that hazardous waves of up to three metres could have been generated by the 7.8-magnitude quake, which struck at 4.38am AEDT at a depth of 49 kilometres.

The initial tsunami warning covered the coasts of the Solomon Islands, Vanuatu, Papua New Guinea, Nauru, New Caledonia, Tuvalu and Kosrae.

But three hours after the quake struck, the PTWC said: "Based on all available data, the tsunami threat from this earthquake has now passed."

"Minor sea level fluctuations of up to 0.3 metres above and below the normal tide may continue over the next few hours," the centre said. Residents in coastal areas should "remain observant and exercise normal caution near the sea," it said.

A 12-centimetre tsunami wave was observed off the coast of Honiara following the quake, the PTWC said, while a 10-centimetre wave was observed off Luganville in Vanuatu.

The quake struck 63 kilometres south-west of the Solomon Islands town of Kirakira. It was almost immediately followed by a 5.5-magnitude aftershock.

The US Geological Survey initially reported the quake was of magnitude 8, but later downgraded its size to 7.8.

There was no threat to the Australian mainland from a tsunami, the Bureau of Meteorology said.

The New Zealand Ministry of Civil Defence initially issued a marine and beach threat, but later cancelled that warning. However, the ministry warned people to stay out of the water due to possible strong and unusual currents for the rest of the day, especially on New Zealand's west coast.

Few reports have come in from the Solomon Islands, where the earthquake has reportedly caused widespread blackouts, knocked out phone systems and even caused the tsunami warning system to go down.

Loti Yates, from the National Disaster Management Office in the capital Honiara, told the ABC that there had been reports of houses collapsing due to the quake.

Mr Yates said poor communication infrastructure had made it hard to assess the immediate impact.

"[One community] that has been in touch with us said they are calling from up the hills, which is good, they activated their own emergency plans and know what to do, so we are happy," he said.

"The warning has been issued ... at this stage, communication difficulties are hampering our ability to get clear information."

One person, Tali Hong, wrote on Twitter from the capital Honiara that there was a blackout across the city.

The PTWC initially issued a tsunami watch alert for Hawaii, but that was later cancelled.

"Based on all available data, there is no tsunami threat to the state of Hawaii," the PTWC said.

Three hours earlier, a magnitude 6.5 earthquake struck about 160 kilometres off the Northern California coast, at a depth of about 12.5 kilometres, the USGS said.

The PTWC said that earthquake, originally reported to have a magnitude of 6.8, was not powerful enough to generate a destructive tsunami. No damage or injuries were reported.

The Solomon Islands earthquake is tied for the strongest in the world in 2016, along with the Ecuador, New Zealand and Indonesia quakes. More than 650 people died when the Ecuador earthquake hit on April 16.

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- with Reuters