A woman who died in the police Booze Bus shortly after failing a drink-driving test is believed to have died of natural causes.

Hamilton woman Elizabeth Jill Gilbertson, 56, died in the arms of police in Hamilton late on Sunday.

She was stopped on Cambridge Road about 10.15pm at a routine alcohol checkpoint, where she failed a breath screening test, police said.

She went to the Booze Bus to complete an evidential breath test procedure but while doing this fell from her chair and appeared to need help.

An ambulance was called and, despite getting immediate first aid, she died at the scene.

A post-mortem on Monday indicated she had died from natural causes, Waikato District Crime Manager Detective Inspector Chris Page said.

He expressed the police's condolences to Ms Gilbertson's family.

Det Insp Page said Ms Gilbertson's death was very upsetting for her family and for police staff at the scene.

Her death has been referred to the coroner, while police and the Independent Police Conduct Authority are also investigating.