"I opened the phone as there were messages showing on the outside screen. When I opened the phone, I noted that it opened to a text message that was half-written."
Mr Troy said he read the received and sent messages, and it appeared a text conversation had been going on.
He said the wet roads did not contribute to the crash.
Coroner Wallace Bain asked how many text messages Ms Jessep had sent in the 45 minutes it would have taken her to travel from Rotorua, her starting point, to Tirau.
Mr Troy said he counted 19, the last sent about 15 minutes before the crash.
Dr Bain said that while she might have pulled over to send some of the messages, the number showed she had been sending texts while she was driving.
"We know she was texting and driving but whether she was at that particular stage."
Two young women injured in the crash have started an anti-texting and driving campaign.
Ms Jessep's son Eliot Jessep, 17, said he had turned down several requests to join it.
"I'm not thrilled about it because it puts my mum in a bad light."
He said he believed his mother could have been distracted by several things, and it wasn't necessarily texting that caused her death.
"What stood out to me was how the heater was sitting. This may not sound like a big thing but if she was driving in a hot car, that would make her a lot more tired and distracted.
"She was in Rotorua earlier in the week for a funeral and she would have been very tired."
Eliot Jessep said he did not think anyone could be 100 per cent sure texting and driving had caused his mother to crash. He said the cause of death should be noted as being "distracted in general".
Dr Bain reserved his decision.
There have been 28 phone-related road deaths since early 2007.
Last month, the Herald counted 29 people in the space of an hour using phones while they were driving north through Auckland's 80km/h central motorway junction.
Anyone using a handheld cellphone while driving can be fined $80 and get 20 demerit points.
TEXTING DEATHS
May 2006: A man dies after crashing car into a power pole. Coroner later finds he was distracted by a series of text messages.
September 2008: A Mid-Canterbury teenager was allegedly sending and reading text messages before his sports utility vehicle crashed into a car, killing two people.
March 2010: A 21-year-old man admitted being distracted by a text message when he drove into and killed a 93-year-old woman on a Hastings pedestrian crossing.
July 2010: A Rotorua teenager denies police claim he was texting moments before a crash in which 11 people were hurt, one fatally.
