The number of confirmed flu cases in Canberra during 2019 has spiked more than 500 per cent compared with the same time last year.
It comes as ACT Health authorities say they don't know why the start to this year's flu season has been so severe.
New figures have revealed that 738 people were diagnosed with influenza in the year to June 9, up from 137 cases in the same period in 2018.
The number of flu cases this year is almost double the number of influenza diagnoses in the ACT for the whole of 2018, with 476 confirmed cases last year.
Acting chief health officer Kerryn Coleman told ABC radio health authorities were surprised by the large increases in cases during this year's flu season.
"We're not really sure why it's the case," Dr Coleman said.
"It may be something to do with a low season last year, and we may be less immune, so we're exposed earlier this year.
"It also may be due to increase travel and [coming back with the flu from] the northern hemisphere winter. We're just not sure."
The rise of flu cases this year in Canberra comes as other states and territories also record a high number of diagnoses.
Dr Coleman said the spike in flu cases in the start of the season could lead to a drop in numbers later in the year.
The usual peak of the flu season happens around July and August.
"We're seeing a spike in cases much earlier than normal," Dr Coleman.
"From a severity perspective, I don't know if this is a more severe season or not.
ACT health authorities said fewer than five people had died from the flu in Canberra this year, but did not confirm an exact number, citing privacy concerns.
Dr Coleman urged for Canberrans to be vaccinated against the flu this year.
"We're particularly focused on people at high risk of influenza-related complications," she said.
"These are people aged over 65 or under 5, pregnant women, Aboriginal and Torres Strait Islander people and people with chronic conditions."
An extra 400,000 flu vaccinations are set to arrive in Australia following the severe start to the flu season and additional demand for the vaccine.
Six million doses of the vaccine are available for free for eligible people through the federal government.