A Ministry of Health audit last September identified no areas for improvement at the rest home, and cited positive reports from residents and families.
But in the January 2019 incident, Lehmann experienced pain one day in her shoulder and her breast.
At 3am the next day, January 4, Lehmann was lying on her bed and panting with her tongue out of her mouth, the HDC report said.
At about 3.30am, a concerned caregiver called the on-call registered nurse, Karen Forbes, at her home.
Forbes, who also managed the home, told the caregiver to record Lehmann's blood pressure every hour and call back if her condition worsened.
Deputy Commissioner Rose Wall said these instructions to the caregiver at 3.30am were poor.
The nurse and caregivers discussed Lehmann's condition twice more but Forbes did not assess Lehmann in person.
"By 9.30am, medical intervention was required, and the nurse did not provide it herself or arrange for it to be provided," Wall added.
At 11.45am, Forbes became concerned about Lehmann's blood pressure and instructed a caregiver to call a GP.
But staff at the rest home did not receive a response from the GP, and did not attempt to find the GP or contact Forbes, so no GP came to see Lehmann.
Wall said for four hours and twelve minutes, no further attempts were made to get urgent medical assistance.
Lehmann's son and two of her grandsons were concerned and in contact with the rest home during the day.
At 3.29pm, John Lehmann called an ambulance.
Forbes later voiced displeasure about that.
Lehmann told the Herald the nurse asked him: "Who the f**k rang an ambulance?"
"I was shocked. I really was," he said.
At 4.06pm Dawn Lehmann was taken to a public hospital, the HDC report said.
She was treated for pulmonary oedema (excess fluid on the lungs) and a myocardial infarction (heart attack).
She was later discharged to another aged-care facility.
The rest home was found to have breached part of the Code of Health and Disability Services Consumers' Rights.
"The report highlights the importance of interpreting cardiac symptoms accurately
and responding to the symptoms appropriately," Wall said.
She said in Lehmann's case, the procedure for getting a GP's help was inadequate, and the nurse's workload and performance were not monitored effectively.
"The caregivers did not recognise the seriousness of [Lehmann's] condition, and failed to take steps to obtain urgent medical care."
The HDC found the home's emergency policy at the time was outdated.
Wall said during the afternoon of January 4, Forbes did not attend Lehmann to assess her, or call the home to monitor her condition.
Wall said the nurse admitted her phone manner with John Lehmann was unprofessional.
The Deputy Commissioner recommended she attend training in cardiac management and in communication with family members.
Wall also suggested training in the responsibilities of a sole registered nurse at an aged-care facility.
Bucklands Beach Rest Home has developed a plan for professional supervision for the nurse, the report added.
Forbes, through lawyer Andrea Lane, said she'd taken further training in communication with family members, just as Wall recommended.
She told the Herald: "Buckland's Beach Rest Home maintains the staff-to-resident staffing ratios required by [agreement] with Counties Manukau District Health Board."
Forbes provided the HDC with an apology to Lehmann's family.
Forbes and Lane also noted that Wall exercised her discretion not to name the nurse in the published opinion.
An HDC spokeswoman said generally, providers were only named if public safety concerns arose, providers frequently breached the relevant code, or refused to comply with recommendations.
But the HDC has no legal power to order name suppression, and Lehmann said his mother would be "horrified" at the case being anonymised.
Lehmann was concerned the HDC, despite a stated commitment to transparency, withheld publication of the rest home name and other details when releasing its report.
"If something's crook, people are entitled to know."
But Lehmann praised the HDC for what he called a thorough investigation.
Dawn Lehmann died in April 2019, aged 91.
Her fight for justice over asbestosis made front page news in 2004.
She took on the challenge after her husband Ross Lehmann died of the workplace-related disease and ACC challenged payments to asbestos victims and families.
"She stood up for her rights, and his," John Lehmann said.
"She was a fighter."
