"I leaned in and touched him, he reacted.
"I kept asking him questions - if he could hear me, where was his pain?"
"Sometimes he muttered something, he had obliviously suffered a very bad concussion."
Mr Hamlin then ran back to the main road, about 1km, to open up all the gates for the approaching emergency vehicles.
"It all happened very quickly, but thankfully the place was soon crawling with fireman, police and paramedics".
He said the digger was operating alongside a gully, next to a dry creek bed, when it rolled after being refuelled.
"There was uneven ground, but it was by no means on a steep hill.
Emergency workers then worked for more than an hour to free the Mr Simpson.
Hawke's Bay Road Policing Sergeant Clint Adamson said it appeared the machine was driving down the hill and was near the bottom, when the boom on the crane swung around causing it to roll.
"He was trapped for some time and suffered serious back and abdominal injuries."
Mr Hamlin said Mr Simpson was an "exceptional digger operator" and that the excavator wasn't in a dangerous position.
"I've seen diggers go into some pretty crazy places, it was not on terrain that a digger would not normally be able to operate in.
"I go along that area all the time on my bike and and it's fine."
He said he had only known Mr Simpson since he started work on the farm, but said he is an "excellent sort of bloke".
The digger was operating on the farm to create working tracks and fix infrastructure.
Police said the accident will now become a Department of Labour investigation.
Hawke's Bay Hospital spokeswoman Anna Kirk said early yesterday evening that Mr Simpson was in a serious condition in ICU.
