A number of people have been injured in Tonga, with hundreds of families evacuated from their homes after Cyclone Gita barrelled across the island nation.

Armed forces have been out since the early hours of the morning, clearing roads in Nuku'alofa and the main route to the hospital.

Power's expected to be out for weeks and many water tanks have been damaged or destroyed.

At least 800 families are sheltering in evacuation centres in the capital - with many other welfare centres unable to be contacted this morning to have numbers confirmed.

Graham Kenna of the Tonga National Emergency Office told Mike Hosking says after 30 years of working in emergency management, he admits he was frightened in his office.

"[We were] on the third floor on one of the strongest buildings in Nuku'alofa, and it was shaking. It was continuously battered by debris from flying rooves and that sort of thing."

Kenna says the damage is widespread.

"Extreme amount of damage. A lot of the older buildings, the heritage buildings in the town have been severely damaged or destroyed."