Tonga eruption 'hugely concerning': Ardern

New Zealand Prime Minister Jacinda Ardern says images of the volcanic eruption near Tonga are "hugely concerning" and agencies are still trying to establish full communications with the country.

Communications in all of Tonga were cut off as a result of Saturday's eruption, making any assessment difficult, Ardern said on Sunday.

But the defence force and foreign ministry were working to establish what was needed and how New Zealand could help, she said.

Ardern will address the media at 3pm (1300 AEDT) to give an update on the situation.