Investigation into the part played by senior commanders in Anzac Day tragedy is still under review.

Senior air force commanders are under investigation over the Anzac Day tragedy which left three staff dead and another seriously injured - but there is only a month left to lay charges.

The detail has emerged in a letter from the air force to the parents of Ben Carson, one of the men killed.

Andrew and Pauline Carson were told by Chief of Air Force Air Vice-Marshal Peter Stockwell a 20-month-long investigation was still going into the 2010 crash north of Wellington.

The letter sent to the Carsons said the next phase of the investigation "which concerns the role of higher commanders is still under review".


It also detailed the current charge which had been laid against the leader of the three-helicopter formation, Flight Lieutenant Dan Pezaro.

He said the charge related to the decision to continue with the mission even though weather conditions had deteriorated.

The charge did not allege the accident was directly caused by the decision, said Air Vice-Marshal Stockwell. He said the court-martial followed an earlier failed charge against the officer who had authorised the fatal flight.

Air Vice-Marshal Stockwell said the investigation was being carried out under the Armed Forces Discipline Act. The legislation shows time is running out for charges to be laid - it has a three-year statute of limitations which expires a month after the third Anzac Day anniversary of the accident.

Andrew Carson had described Flight Lieutenant Pezaro as a scapegoat for air force bosses who were found responsible for overseeing a poor safety culture.

He said he did not expect further responsibility to be found in the command chain. The military justice system was unable to properly investigate commanders and an external inquiry into the crash was needed.

The air force has faced no external inquiry into the crash. It was being taken to court by crash survivor Sergeant Stevin Creeggan to force an independent review.

The legal action appears to be on hold after an application by the Herald to search the court file was rejected by the Wellington District Court on the grounds of a possible settlement between Mr Creeggan and the air force.