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Evelyn was initially critically injured after the oak fell on to a picnic table where she was taking shelter.
Police said the death has been referred to the Coroner.
Emergency services rushed to the incident at Victoria Square around 10am on Friday.
"Freakishly high winds" swept through the town and locals described seeing mini-tornadoes around the time of the incident
Evelyn was taken to Waikato Hospital but police later said she died from her injuries.
"Police can confirm a woman has died from injuries sustained after being trapped under a tree in Cambridge."
Richard and Trixie Millet had parked their campervan in front of the fallen tree in Cambridge's Victoria Square, possibly just minutes after it hit the ground.
At the time the pair were unaware a person was stuck underneath.
Two trees had fallen but at that stage - around 10am - no emergency services were at the scene and there was no indication someone needed help.
They walked to a nearby cafe, but when Richard returned to the camper, he knew something was wrong by the presence of emergency services staff digging with shovels around the base of the fallen tree, presumably to free Evelyn.
"It was devastating that I wasn't aware that someone needed help, that we could have called emergency services earlier," he told the Herald.
Waipa District Council said the tree was felled by "freakishly" high winds.
The tree was a healthy pin oak that was about 80 years old.
It had been pruned at the start of April and there had been no previous concerns about it, council group manager customer and community services Sally Sheedy said.
"It has been described to me as a mini-tornado which look to have lifted a perfectly healthy tree out of the ground. Our concern and thoughts now are with the person who was trapped and we sincerely thank those who acted so quickly to help."
The death came as bad weather – including strong winds – wreaked havoc in areas of the North Island, including a tornado that hit Levin on Friday morning.