She said it "was a pretty horrendous three years".
"In retrospect I think we had assumed his failing memory was just his age, but it was clearly much more substantial than that."
Ms Prendergast said the family once again received devastating news a month ago - when her mother was diagnosed with vascular dementia.
"Quite suddenly, literally overnight, my mother developed a different type of dementia and was diagnosed last month.
"We have just had to put her into a residential care facility, and we are now facing what could be a long time with mum, with significant cognitive loss."
Ms Prendergast said people needed to understand, and differentiate, the difference between what was a little bit of memory loss, and what was significant memory loss.
"And I think the message is if you know someone, a partner, a friend, a family member, who you think has significant deterioration in their mental ability and memory loss, get them checked out.
"Don't keep thinking this is just normal for their age.
"Then you get more choices... by the time dad was diagnosed, there weren't any choices, and at least with mum, although it's a different sort of dementia, and it was overnight, as a family we had a better understanding of the sort of support we could get for her."
Alzheimers New Zealand's executive director, Catherine Hall, said dementia was one of today's leading healthcare challenges.
"We must use the findings of this report to ensure that services are in place and are properly funded, to fight the stigma that people affected by dementia experience, and to help build a dementia friendly New Zealand."
