The family of a Sydney woman who tested negative for Covid-19 twice before she died have broken their silence.
Ann Fahey, 76, died on Saturday, 48 hours after her third Covid-19 test came back positive.
But her family are devastated after being told by Newmarch House aged-care facility the first two tests were likely wrong.
Ann's son Mark Fahey says the family are angry and believe an earlier positive result could have given her a better chance of survival.
"I'm appalled. I'm angry. I'm upset," he said.
"If the test was correct - in the first place - she would've had a week's head start."
Ann's granddaughter Nicole Fahey said the family only found out about her positive test after an accidental phone call to the care facility.
"Dad accidentally hit a mobile number and he was calling back the nurse who spoke to him earlier that day," granddaughter Nicole claimed on Sunrise.
"She said 'oh, I'm not dealing with Anne Fahey anymore. Haven't you been called yet? She's positive."
"So we actually found out by accident.
"Nan was so happy and completely fine, she was 76.
"In fact, just on Wednesday she told her sister and friends that they didn't need to call her every day because she's fine."
Fahey became the 14th of 15 people to have died from Covid-19 at the aged-care facility, which has now seen at least 63 confirmed cases.
Fahey's negative results have now caused implications at the home with nurses who treated the 76-year-old also treating other residents.
A male resident at the facility, who had no immediate family, also died after testing positive to Covid-19.
The aged care facility has since announced touch health protocols, including testing all staff on entry and separating patients who have tested positive.
In a statement, Anglicare Sydney has described the virus as "devastating" for the elderly in the community.
"The impact of Covid-19 on older people is increasingly evident in Australia and internationally and this is extremely distressing for other residents, their families and members of the wider community," the organisation said.
"Anglicare appreciates the support of the Commonwealth Government in assisting us to source an infection control specialist who is currently on site this week, monitoring and reviewing all current contamination and infection control procedures.
"We also appreciate government support in implementing the Family Support Program, which will enable registered nurses to communicate regularly to families about their loved ones."
Why do some Covid-19 tests come back as 'false negatives'?
New Zealand's first case had to be tested three times before returning a positive result – the first two were from a throat sample, but a third was carried out when symptoms of a lung infection emerged.
Overseas, there have been cases of people having up to six negative results before finally being diagnosed.
Using the diagnostic test developed by the US Centers for Disease Control and Prevention, a negative result meant SARS-CoV-2 wasn't found in the person's sample.
But in the early stages of infection, it was possible the virus might not be detected.
Other reasons for those reported false-negatives might be due to the way those tests were carried out – or that there was a poor genetic match between a section of selected code in the virus, and the virus in the patient.
If a person returned negative results while experiencing symptoms of suspected Covid-19, it likely meant that it wasn't causing their illness.