A post mortem examination has indicated that the death of a 56-year-old Hamilton woman who died in a booze bus in the city last night was due to natural causes, police say.

But they are still working to establish the circumstances surrounding the death of Elizabeth Jill Gilbertson.

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The death was a tragedy for both the dead woman's family and for the police officers in whose arms she died.

"In the first instance police would like to express their sympathies to the family of the woman," district crime manager Detective Inspector Chris Page said.

Her death has been reported to the coroner and investigations were underway, including those by the coroner, police and the Independent Police Conduct Authority.

Police first became concerned for the woman after she had been stopped near a routine police alcohol checkpoint in the suburb of Hillcrest about 10.15pm, Page said.

She was the only person in the car, and had been unable to complete a breath screening test.

"As a result of this, she was required to undergo an evidential breath test in a booze bus at the checkpoint.

"While undergoing this test inside the bus the woman collapsed, fell from her chair, and despite the immediate first aid provided and the efforts of arriving ambulance staff, she died at the scene," Page said.

"Each day and night members of our Traffic Alcohol Group, who operate our alcohol check points, go out with the intent to save lives.

"Last night while they were attempting to do this, a driver has died, literally in the officers' arms.

"In addition to ensuring the investigations being carried out are done so in a professional manner the Waikato Police will be working to ensure those staff affected by last night's events receive the appropriate support as well."

- © Fairfax NZ News