The eruption "obliterated" a volcanic island north of the Tongan capital Nuku'alofa, the agency said.
Tonga says more than four-fifths of the population has been affected by the tsunami and falling ash.
Three people were confirmed killed in the tsunami last week.
Before the eruption, the Hunga Tonga-Hunga Ha'apai volcanic island was two separate islands joined by new land formed in 2015.
Nasa says the eruption was so powerful all the new land is gone, along with "large chunks" of the two older islands.