Emergency workers arrived to find the plane ablaze and the men dead.
Two Civil Aviation Authority investigators arrived at the crash site about midday today to carry out a scene examination.
They removed some parts of the aircraft for closer examination and testing before wrapping up work on the site.
The rest of the wreckage has now been released to insurers.
Pilot Richard Jowsey said the call was heard at the Parakai Airfield.
"He [the pilot] said, 'Mayday, mayday, mayday.' He said his engine had failed and they were going to put down using the emergency landing procedure," Mr Jowsey said.
"That mayday call shook them up to hell at the airfield. It was pretty broken up and scratchy."
The mayday call was also heard by an instructor at the Dargaville Aero Club, where the plane was due to land.
Murray Foster, a friend of the men and their flight instructor, said a colleague also heard the call.
"They said it sounded like the guys, but it was very weak. I called them back on the radio to see if I could contact them ... I couldn't."
Mr Foster said the men had been very excited about their trip away.
They landed in Paraparaumu and stayed with friends for a night, before driving to the Ohakea air force base, near Bulls, for the annual air show.
"It was their own plane. They only just bought it - they'd had it about a month or six weeks. [The pilot] had owned several planes before and he was experienced," he said.
"They did a lot of flight planning; their planning was really good. I helped them with it and they were very particular about their planning.
"They were really good guys ... It's very, very sad."
Mr Jowsey, an experienced pilot who knows the area well and has flown over it many times, suspected that as the plane glided in for the emergency landing, it was struck by a "king wave" of wind.
The area is known for its gusty conditions, he said, and while the pilot appeared to pick the "perfect spot" to land, the wind might have been the fatal factor.
"It was probably a combination of bad weather and bad luck. It's horrible. It would have been a hell of a slam ... It would have been quick."
From what he heard on the mayday call, Mr Jowsey did not believe the crash was the result of pilot error.
"He did all the right things, I think. He's died bravely trying to save his plane and passenger.
"Were it not for the easterly gusty conditions, he would have probably walked away from that."
Investigators will continue to interview witnesses and analyse weather conditions and the intended route of the aircraft.
The findings of the investigation would be released in about a year.
APNZ NZH lb mb gf
