In the third week of September last year intensive care units across Australia struggled to function after being inundated with patients severely sick with the flu.
Professor David Pilcher at The Alfred hospital in Melbourne was working night shifts that "shocking" week and said the ICU was seriously under strain.
"We had a large number of ventilated patients, we had patients on heart-lung machines, we almost ran out of dialysis machines to treat patients with kidney failure," said Prof Pilcher.
It was the busiest ICU he had worked in for years.
A study released on Thursday has now shown ICU admissions in Australia and New Zealand in 2017 peaked at levels not seen since the swine flu pandemic of 2009.
Researchers at Monash University examined all admissions to 181 ICUs between January 2009 and December 2017 using the The Australian and New Zealand Intensive Care Society (ANZICS) Adult Patient Database.
Compared to past seasons, pneumonia and sepsis ICU admissions rose by as much as 50 per cent in 2017 - the highest seen since registry records began in 1993.
This was the indirect consequence of the flu outbreak, Prof Pilcher said.
"The intensive care data doesn't specifically tell you about flu but what it does tell you about is admissions due to pneumonia and sepsis, severe life-threatening infections," he said.
Admissions progressively climbed over the winter period and then peaked the the week beginning September 17.
"That climb in admissions and peak exactly matched the flu notifications," Prof Pilcher said.
"Anecdotally the patients that we were seeing coming in and filling up our ICUs, here in Melbourne and in Sydney and elsewhere around Australia, were patients with flu and with infections they got as a consequence of flu," he said.
At the peak, pneumonia or sepsis accounted for 16 per cent of all ICU admissions.
In 2017, a record 1100 influenza-associated deaths were recorded, of which 90 per cent were elderly patients.
The flu virus can directly affect the lungs but also makes the body more vulnerable to other infections.
"Because you are already sick when you get that infection you are more likely to get sepsis, which is the development of organ failure," explained co-author Dr Aidan Burrell.
"Why one person gets it and another person doesn't is difficult to say," he said.
But it's not just old people who are at greater risk, he warned.
"We saw many young people, particularly here at The Alfred, with very severe sepsis," Dr Burrell said.
He hopes the study will better prepare ICUs for similar flu seasons and encourage people to get vaccinated.
"Vaccination is a really important part of prevention of these outbreaks," Dr Burrell said.
Staying at home when sick and hand washing are also critical to prevent the spread of flu, he said.
"If everyone does that we can really prevent another flu season like last year."
