The speed limit on the road was 100 km/h and the weather was fine.
As Paul approached a moderately tight downhill corner, the unladen truck veered to the left-hand grass verge and crossed the centre line into the right-hand lane, Coroner Dunn said.
The truck then crossed the road, went through a fence and travelled about 130m across a paddock before dropping over a grassed bank and down a steep hill towards the canal.
Dunn said there was no evidence the truck had braked, slowed or attempted to stop, and it remained upright as it entered the canal and quickly became submerged.
The Ruahihi Canal carries water from Lake McLaren to the Ruahihi power station.
Emergency services attended and the canal water levels were lowered to obtain access to the submerged truck.
Coroner Dunn said the recovery operation, which included a crane and assistance from the police dive squad, took several hours and Paul was found inside the truck.
Police and emergency services at the fatal crash site after a Downer's truck driven by John Riki Paul ended up submerged in Ruahihi Canal. Photo / NZME
The truck was extensively damaged however no faults were found that could have contributed to the accident, she said.
A post-mortem confirmed Paul had suffered a sudden ischaemic cardiac event which led to the loss of control of the truck as it headed down the slope and into the canal.
"Paul previously suffered from high blood pressure, high cholesterol and gout. He was on prescribed medications for these conditions. He regularly attended his doctor for check-ups.
"He kept a regular work schedule, was well-rested and took regular breaks as required."
Coroner Dunn also said Paul had not been drinking alcohol or consumed drugs.
The post-mortem also revealed Paul suffered a fractured cervical spine, however, this injury had happened after his death and was likely to be from travelling over the rough terrain.
"There was no evidence of drowning, indicating that Mr Paul died from the cardiac event prior to entering the canal."
Coroner Dunn said Paul died from "natural causes" being a sudden ischaemic cardiac event, associated with severe coronary artery atheroma and hypertensive heart disease.
The truck crashed through a fence before heading into the canal. Photo / NZME
The Coroner said she was satisfied there were no circumstances relating to Paul's death that would make an inquest inquiry necessary or desirable.
At the time of the crash, water was drained from the Ruahihi Canal to access the truck. This took about two hours, at about 100mm per 15 minutes, and was done so the vehicle could be accessed safely.
Mount Maunganui Surf Lifesaving was called by police to use the club's inflatable rescue boat.
The retrieval operation took about three hours as the vehicle was completely submerged in the canal.
