"I thought 'oh no' [but] I could see they were getting out fortunately," he said.
Dargaville Volunteer Fire Brigade chief fire officer Jeffrey Palmer said a perfect landing meant the pair escaped relatively unscathed.
He said if the pilot had gone a couple of hundred metres further he would've landed in a flat paddock but he just happened to hit a hilly paddock, then a fence post.
Mr Davidson said he stopped the digger at the top of the hill and walked down to check the couple were okay.
He said the man had a "pretty sore back" and the woman had a cut on her head. They were both taken to Dargaville Hospital by ambulance.
Mr Davidson understood the pair, who were from Parakai, 43km northwest of Auckland, realised there was engine trouble and had turned to head to the Dargaville Aerodrome when they crash landed.
He said the man had called emergency services but handed the phone to Mr Davidson to describe where the farm was.
Police, ambulance and four fire trucks arrived at the scene "pretty quickly", he said.
"We're quite used to having cars come through the road fence but a plane coming through the fence is a different story," he said.
Mr Davidson said fire trucks remained at the scene for about an hour as fuel was leaking.
The Civil Aviation Authority has been notified but were awaiting more information before making a decision about whether to investigate or not.