The woman said she watched in her rear-vision mirror as a ute behind her and the woman's vehicle then collided head-on.
''The impact was extremely hard."
The woman stopped and ran to the crash to help. "Although I won't go into details of what I saw when I approached the vehicles out of respect for the families, it was incredibly serious."
Initial reports said three people had been seriously injured and one had sustained moderate injuries in the crash, however, one person died shortly afterwards.
Two of the injured were airlifted to Waikato Hospital with the third flown to Tauranga Hospital.
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A person at the scene said the road had been littered with what he believed to be nails from the ute. He described the crash as "ugly".
A section of SH30 was closed until mid-afternoon while a scene examination was carried out.
Yesterday's death brings the Bay of Plenty road toll to 64 for the year, up 13 on last year, and is the highest police district toll in New Zealand this year to date.
Bay of Plenty Police Inspector Brent Crowe said the toll included five multiple fatality crashes that caused the death of 24 people.
"One death on our roads is one too many," Crowe said. "This year 64 people started a journey on a Bay of Plenty road and did not make their destination.
"The sudden and completely unnecessary death of these people has left a huge void in the lives of those they have left behind."
He said, in the lead up to the summer months, police were imploring road users to remain focused and drive safely, refrain from alcohol and drugs, drive within posted speed limits and to the conditions and ensure everyone in the car wore a seat belt.
Breakdown of the 64 deaths on Bay of Plenty roads this year:
Western Bay: 13 fatalities from 10 crashes
Eastern Bay: 14 fatalities from 12 crashes
Rotorua: 7 fatalities from 7 crashes
Taupō: 30 fatalities from 13 crashes