An Air New Zealand flight to Tonga has been forced to turn back - meaning a third day of disruption for Kiwis travelling there.

The airline put on an extra flight from Auckland to Nuku'alofa this morning, after several days of volcanic activity which has seen flights diverted and cancelled.

Flight 6868 took off this morning, but has now turned back, as volcanic activity continues.

An airline spokeswoman says it means the flight from Tonga to Auckland - flight 6869 - has also been cancelled.