Sydney - A 7.4-magnitude earthquake struck 255km southwest of Tonga on Friday, according to the US Geological Survey, but there were no reports of damage.
The quake hit at 05:19 on Thursday, and was centred 282km southwest of Tonga's capital Nuku'alofa and 171km deep.
It was followed four hours later by a second powerful 6.6 tremor 84km northwest of the town and 103km deep.
"A destructive tsunami was not generated based on earthquake and historical tsunami data," the Pacific Tsunami Warning Center said of each quake.
The Matangi Tonga website said the quakes "shook homes and rattled windows" in Nuku'alofa but reported no damage.
The powerful tremors also caused false alarms in New Zealand where the official GNS Science monitoring system reported tremors of 6.1 and 5.9 magnitude off the east coast of the North Island.
A GNS spokesperson said the New Zealand measuring devices had picked up the quakes from outside their network and tried to locate them and automatically sent out alerts, which were later deleted.
The two quakes off Tonga came 12 days after a similar 6.5-magnitude earthquake struck near the pacific nation at a depth of 205km.
Tonga, almost 2 000km northeast of New Zealand, lies on the so-called "Ring of Fire", where continental plates collide causing frequent seismic activity.