A crowd of onlookers watched as volunteer firefighters smashed the truck windscreen to get to the trapped man.
Ōhope brigade officer in charge Mike Christini said when crews arrived a woman who witnessed the crash was comforting the injured passenger.
"A member of the public was lying down and had reached underneath to hold the person's hand and was talking to him when we got there, so she was doing a great job."
The Ōhope brigade were helped by a team from Whakatāne as emergency personnel dug underneath the cab to make room for the air bags that would free the man's arm.
"We removed the windscreen first so that the medical team could have access to the patient and administer treatment and pain relief," Whakatāne station officer Chris Hands said.
"And we dug airbags under the cab of the truck and used the airbags to lift the truck off the patient's arm,"
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Hands said it was a lengthy process, taking around an hour to extract the man from the truck due to the airbags only expanding six inches at a time, and needing to reset the devices multiple times.
While they were working, one of the youngest, and smallest, Whakatāne firefighters climbed into the truck to comfort the passenger, keep him calm and advise him of what they were doing.
Christini said St John staff assisted with helping the man out of the truck to ensure he suffered no further harm.
"It was a real team effort and lots of people were involved."
Branch debris and grass were torn up as the truck, which was carrying a large load of milk bound for local shops, hit the reserve.
A bystander told the Beacon that this corner at the bottom of the hill near the West End turn-off was a hazard with people not wanting to slow down for the sharp turn.
Eastern Bay road policing manager Senior Sergeant Chris Howard said police spoke with three eyewitnesses at the scene and had begun an investigation into the cause of the crash.
"We don't know the cause of the crash at this stage, we're investigating this now. The truck is being examined for any faults, none of the milk fell out and the council has been informed of the damage made to the bollards," Howard said.
Howard said police would examine forensics on the road and investigate any environmental, physical or driver issues that might have caused the crash.
"It was lucky nobody was seriously injured," he said.
The injured passenger was first taken to Whakatāne Hospital by ambulance and then transferred to Waikato Hospital for treatment.
A Waikato District Health Board spokesperson said the 28-year-old man remained in a stable condition after suffering severe arm injuries.