As well as this, those under the age of 65 with specified medical conditions, pregnant women and children under the age of five with a history of significant respiratory illness were also vulnerable to influenza, the study found.
Most of those who died with influenza were killed by other illnesses that were triggered by the influenza infection, study co-author Professor Michael Baker said.
He said these illnesses included bacterial pneumonia and heart attacks.
The study showed the need for more people to immunise themselves against influenza, he said.
"What this research shows is that if we could get rid of influenza we would avoid those 400 deaths each year, in many cases where people are very elderly and frail.
"These findings reinforce the need to continue efforts to develop and implement effective measures to prevent influenza and reduce the harm it causes to vulnerable groups."
Ms Leung said she was unwell for a week or so with what she thought was a "bad cold" but then was so unwell her mum took her to Middlemore Hospital.
"Within two hours I was in the intensive care unit struggling to remember my own name.
"It is quite frightening when a week before you were just a normal 20-year-old and then suddenly you can't control your own body."
Ms Leung said she deteriorated quickly, and was soon put on a ventilator to help her breathe.
Since recovering, Ms Leung has been the face of an annual influenza vaccination campaign, and continues to encourage people to immunise themselves against the disease.
"When you're 20 you're normally fit and healthy and nobody thinks you are going to get that unwell from the flu.
"When I got unwell I hadn't been immunised - I didn't even realise you could get the immunisation then.
"Obviously now I realise how important it is for myself and for everyone else to do what they can to protect themselves and people around them from getting the flu."
