It said large doses could cause life-threatening conditions, such as renal and liver failure, heart attacks, haemorrhages, strokes and seizures.
Mr Smith ruled that Ms McMillan died as a result of an intracerebral haemorrhage with 0.24mg of the drug in her system per litre of blood.
New Zealand Drug Foundation executive director Ross Bell said Ms McMillan's death would be the first reported methamphetamine overdose.
"If [the coroner] has ruled that it's a meth-related death, then yes, it would be the first reported overdose."
It is not clear how long Ms McMillan had been taking the drug.
People who knew her said she was hard-working and dedicated, happy and healthy, admired and respected.
Everything appeared to be normal in her relationship with her fiance and she had a close and loving extended family.
A family spokeswoman said her death was an "absolute shock for all of us. We are all quite shocked because certainly she was an amazing person, an absolutely amazing person.
"She had a very, very good job, an amazing home. It was just completely out of character and a very peculiar situation," she said.
"We don't know, we just don't know any more than that really."
AXA declined to comment, but former chief executive Ralph Stewart, who worked with Ms McMillan, said her death had been difficult for her colleagues.
"When we lost Lisa it was an absolute tragedy. She was a wonderful person, a very big part of AXA.
"The measure of the relationship Lisa had with AXA and the business - the funeral was almost equal family and AXA people."
Asked if staff noticed any unusual behaviour before her death, Mr Stewart said: "Absolutely, completely and totally none whatsoever."
Ms McMillan and her fiance had lived in a two-storey home with Wellington harbour views near the end of a leafy cul-de-sac in suburban Fairfield.
A neighbour said the couple kept to themselves and never caused any problems.
"It's such a horrible thing, and ordinary people are taking it," she said.
Mr Bell said there was not a typical methamphetamine user.
"I'm not surprised that a corporate is using meth.
"When it first hit the scene it was certainly a drug that was first used by the bikie gangs and the dance community, but then it quite quickly moved across all parts of society," he said.
"In fact it's a very expensive drug and so we do see it in the corporate ... world."
National Addiction Centre director Doug Sellman said methamphetamine use in New Zealand had plateaued in the past five years after rises of epidemic proportions in the late 90s.
Thousands of New Zealanders took it from time to time, but the majority of users were not addicted.
