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The head of Western Bay road policing, Senior Sergeant Ian Campion, said the preliminary crash report indicated weather, speed, alcohol and road conditions were not contributing factors, and the investigation now centred around two main issues.
"We believe Ms O'Brien was manipulating her cellphone while driving and also believe she was not wearing her seat belt. The preliminary indications are these factors contributed to the crash."
Mr Campion said he was "absolutely gutted" at the number of fatal road crashes this year.
He said there had been six fatal crashes on Western Bay roads this year, the same number for the whole of last year. Apart from one, which involved a medical condition, every one of the other 11 crashes were "absolutely preventable".
"It's very frustrating and hugely disappointing for police and our road safety partners. We [police] take it very personally when there is even one serious injury crash or fatality as we are out there every day trying hard to make sure people are able to get to and from the Western Bay safely," he said.
In 2013, there were 867 crashes across the district, 50 resulting in serious injuries and six deaths, and so far this year there has been 279 crashes, including six deaths and 25 serious injury crashes.
Mr Campion said police could only do so much and people needed to take responsibility for their own driving and heed the warnings about the "fatal five" factors which led to crashes - speed, alcohol, not wearing seatbelts, using cellphones and other distractors, and non-compliance of the rules at intersections.
Ms O'Brien died when her car crossed the centreline and crashed into a people mover at 12.30pm on Saturday.
Her two children Faith, 3, and Phoenix, 4, were injured in the crash, and remain in Starship Hospital.
Phoenix is recovering from major surgery to repair his broken pelvis and thighs and also broke his arm, while his sister had a minor head injury.
- Bay of Plenty Times