Police have referred the incident to the coroner.
John Sinclair, executive officer of the New Zealand Helicopter Association, said Mr Robb was a "very experienced pilot, well-respected in the industry".
The chopper had been spraying farmland in the remote rural area, close to State Highway 3 at Kai Iwi, police said. It came down on Koatanui Rd, in steep hill country.
The Fire Service has been clearing fuel spilled across the road and investigators from the Civil Aviation Authority (CAA) were expected to arrive by mid-afternoon. The accident scene has been cordoned off. A spokesman said it was too early to comment on how the crash might have happened.
Residents heard the helicopter working in the area this morning.
Wanganui Chronicle reporter Zaryd Wilson said the crashed helicopter was "still relatively intact''.
Weather in the area was fine. "It's a great day, no wind. It was really foggy earlier this morning, but it would have lifted by [the time of the crash]."
The helicopter was a Hughes 500E model, a light utility chopper, which can hold up to five people, and is 9.4m long.
Mr Robb was a keen motorsport enthusiast and raced V8 utes for nearly a decade.
In April, he won his first UDC V8 utes race at the ITM 500, a support event to the V8 Supercars at Pukekohe.
In 2012, Mr Robb also he raced at Mt Panorama ahead of the 50th anniversary of Bathurst.
- additional reporting Zaryd Wilson of the Wanganui Chronicle