Investigators have found the black box flight recorders of an Air New Zealand Airbus A320 which crashed in France last week, but they still cannot explain how the incident occurred, French authorities said on Wednesday.

The plane, which had been leased to a German carrier, was being refitted and tested before returning to Air New Zealand, when it plunged into the Mediterranean sea on an approach run into the southwestern city of Perpignan on Nov. 28.

All seven people aboard are believed to have been killed.

France's BEA civil aviation security organisation said the aircraft's flight recorders have been found and their protective casing and memory cards appear to be intact, but investigators have so far been unable to extract information from them.

"Additional work is needed although it is not possible at the moment to predict results," the authority said in a statement.

"The crew had given no indication of any problem to air traffic control when it stopped responding to calls," the statement added.

"At this stage of the inquiry, nothing explains why the aircraft left its trajectory and crashed into the sea."

The A320 is a twin-engine, single-aisle airliner made by the Airbus unit of European aerospace group EADS that normally seats around 150 passengers. About 1,960 A320 aircraft are in service with airlines around the world.