Pilots have been warned to take caution when flying through airspace near the volcano after the aviation alert level for the Home Reef was raised to orange, one step below an emergency.
Mariners have been told not to sail closer than four kilometres from Home Reef and to look out for falling ash. Seismic activity is fairly common around Tonga, a small archipelago nation with about 100,000 people spread across 171 islands.
In January this year, it was rocked by the eruption of the Hunga-Tonga-Hunga-Ha'apai volcano, which was one of the most powerful in modern times, according to researchers from the University of Bath.
The blast, which felt "like an atomic bomb" according to local aid workers, created a 15-metre tsunami that killed three people, destroyed villages and severed Tonga's communication cables, leaving the country cut off from the world for weeks.