It appears he got out of his cab to check why the large spinning blade on the tall machine wasn't cutting as it had earlier.
Two workmates rushed to Mr Spackman's aid and emergency services were called to the property, 4km from Waiau township in the Hurunui district.
St John Ambulance, police and a doctor also rushed to the scene but could not save his life.
Property owner Graeme Phillips, 64, was working just up the road when a vet called on him to say something had happened at his place. He raced home to find emergency services and distressed workers at his front gate.
The local police constable looked shaken as he told Mr Phillips he had better go inside, as there had been a terrible accident.
Last night, he still could not come to grips with what had happened on his land.
"I've got no idea how it happened, and I don't want to know either," Mr Phillips said.
"All the speculation in the world won't bring him back, and tonight two wee kids won't have Dad come home. It's a total tragedy."
The image of Mr Spackman lying dead on the grass in the shade of the hedge would haunt him "for the rest of my days", Mr Phillips said.
Mrs Spackman and her husband were childhood sweethearts who met at Katikati College.
They moved to the Hurunui district 15 years ago, where Mr Spackman, a "cheeky chappy" with a dry wit, enjoyed rugby, hunting, fishing, and camping. He also enjoyed restoring classic American Hudson motorcars in his shed.
But since his sons came along, "family was his whole world", Mrs Spackman said.
While admitting she was still in shock, she believed she "will be fine, eventually".
"As for the wee one..." she tailed off, before breaking down, hugged by her mother-in-law Karen.
Occupational Health investigators from the Ministry of Business, Innovation and Employment were at the scene before Mr Spackman's body was taken away.
The death has been referred to the coroner.