"The vehicle has become airborne and struck the opposite side of the bank, before rolling into the Karamu Stream," Senior Sergeant Greg Brown said.
It's understood Mr Hamahona survived the initial impact of the crash and was able to make one last phone call to family.
"His father and another family member have tried to rescue him from the vehicle before contacting emergency services," Mr Brown said.
Police arrived on scene about 4.50am to find the vehicle submerged with the driver trapped inside. He was the sole occupant and died at the scene.
Heavy lifting equipment was used to pull the crumpled wreck from the stream, closing the road for about five hours.
At the time of the crash the weather conditions were good, and the Police Serious Crash Unit would investigate circumstances which led to the crash.
Speed, distraction and alcohol would be a focus, as the crash occurred on a moderate bend and the driver is believed to be familiar with the road.
"This is obviously a tragedy for the family, and a really disappointing start of Road Safety Awareness week. No one should lose their life travelling to work, and police would encourage drivers in general to watch their speed, pull over safely to answer any cell phone call, and drive to the conditions", Mr Brown said.
"Our staff will continue to be out in force focussing on these key contributors to crashes, so I encourage everyone to drive responsibly and keep us all safe."
Drivers approaching the bridge from the western side (i.e. travelling toward Havelock North) were urged to be cautious until the armco barrier had been replaced.
- Additional reporting APNZ
