Emails released under the Official Information Act show our main transport watchdog, the Transport Accident Investigation Commission (TAIC), has been criticised over its reports into crashes involving the Robinson helicopter.
The criticism came from the helicopter company itself and surprisingly, our other air safety agency, the Civil Aviation Authority (CAA), appeared to agree. In the emails TAIC is accused of "not being fair" and having "outbursts" in safety reports.
The emails in question refer to a particular kind of event known as 'mast-bumping', when the rotor blade crashes through the cockpit
TAIC says that's killed 18 people in the past 20 years.
It's been trying to improve the aircraft's safety but most of its recommendations haven't been implemented by the CAA.
Duncan Didsbury's brother Mark was killed when his R66 Robinson broke up mid-air. Four years on, he still has major concerns the recommendations of TAIC haven't been implemented.
"Given the disproportionate amount of accidents that are happening in Robinsons and the same type of circumstances being exhibited time and time again ... more work needs to be done," Mr Didsbury says.
TAIC wants special training for R66 pilots and a renewed investigation into the design of the Robinson's main rotor - but so far neither the CAA nor the US regulator has acted.
Fourteen fatal crashes in the past two decades have been put down to mast bumping and the outcome is usually catastrophic.
"As in Mark's case he was flying within the manufacturer's guidelines and something catastrophic happened. I just don't know how that can be okay," Mr Didsbury says.
David Yeomans, an independent aviation expert and pilot, agrees.
"In my opinion, this aircraft has a rather unique propensity to destroy itself inflight."
But he also has concerns about the email chain obtained by Newshub - where TAIC's safety reports are described as an "outburst".
"It's an inference that they've overreacted and anything published by an agency is not an overreaction - it is a reaction to considered effort and information and they have formed an opinion," Mr Yeomans says.
He says it's inappropriate.
"It has no place in a professional relationship."
The reason he's concerned is because the emails are between our Civil Aviation Authority, the agency that makes rules around aircraft, and the maker of the helicopter in question, Robinson.
In August last year, an instructor for the Robinson Helicopter Company told the CAA's Andrew McKay: "We are preparing a response to TAIC's recent outburst".
Mr McKay replied: "What outburst are you referring too (sic).. the last I am aware was the draft report for (REDACTED) accident. Have you got another?"
The Robinson representative responds: "It was a TAIC watchlist. It is certainly not a fair assessment."
In a written statement, the CAA told Newshub the emails were just "banter" and do not reflect in anyway what CAA thinks of TAIC as an organisation.
It says it respects TAIC's role as an independent investigator.
But not everyone's convinced their relationship is functioning well.
Mr Yeomans has spent eight years in New Zealand and 15 years in Australia working on aviation safety.
"If language like that is being used in emails, it is certainly an indicator that not all is well," he said.
As for Mr Didsbury, he's satisfied TAIC made the right call putting the Robinson on its watchlist. He says what's needed now is action.