The inquiry, by Wellington lawyer Matthew McClelland, began in about April 2011. It was due to be completed in December when the terms of reference were extended by the air force to include input from families of others killed in the crash.
The Herald understands those families were happy with how they were dealt with by the air force. The deadline was extended to July but still was not completed.
Mr Carson said the family had recurring problems with the way they were treated. The most recent was discovering on Anzac Day this year that service medals had been presented to their son posthumously.
He said the couple found the medals had been given to Ben's partner. Mr Carson said the air force did not tell them Ben had been awarded medals.
Chief of Air Force Air Vice-Marshal Peter Stockwell said two sets of medals were given to Ben's partner, with the second set intended to be handed on to his parents.
"The NZ Defence Force was only obliged to provide one set. However, the second set was paid for out of air force non-public funds ... in the interests of Ben's parents as well as Ben's partner both retaining copies of his medals."
Defence Minister Jonathan Coleman said he had been briefed on the reasons for the delay and accepted they were valid.