Villagers should refrain from diving and fishing in coastal waters following reports of submarine rumblings and what sounded like explosions, the NDMO said.
People should also avoid the mountain's slopes, the office said, describing the situation as "high-risk".
The NDMO and the Mineral Resources Department were despatching an assessment team to the area today, a spokesperson said.
A full media briefing is expected later today.
The warning follows a magnitude 5.1 quake in the area on Sunday, which has been followed by multiple aftershocks.
Nabukelevu's last recorded eruption was in 1660.