Marco Polo Aged Care Services has placed its Waples Rd facility in lockdown until at least the end of the week after around 20 residents, and 20 staff members, fell ill with flu-like symptoms.

Five residents were hospitalised - three of whom died - while the 15 residents remaining at the facility are being kept isolated to stop the spread of infection.

"This flu is a killer," Marco Polo manager Robert O'Shea said. "My plea would be for people to please get the flu shot to protect themselves, and others.

"I'd also urge people that if they do have a relative in an aged care facility, and they themselves have flu symptoms, to please stay away from that facility.

"If they do bring the infection in, they're going to be putting at risk people who are very vulnerable and susceptible to complications."

Mr O'Shea said that the outbreak started on the June long weekend. Relatives had quickly been notified and the facility had since been working with the public health unit to control the infection.

"There's a process we follow when people have the flu - we contact their families and give them Tamiflu tablets, and it usually takes around five days for the illness to run its course," he said.

"When there's an outbreak we try and isolate residents into pockets - so those with flu symptoms are kept together - staff movements are minimised across the facility, and we take a number of other precautions to control the outbreak.

"Using such measures at Unanderra we have been able to contain the flu outbreak to our Unanderra Care centre, which houses 80 residents, and it hasn't affected our Cordeaux Lodge which has a further 80 residents."

Illawarra Shoalhaven Local Health District public health director Curtis Gregory said there had been 21 flu outbreaks in residential care facilities throughout the district this year.

Meantime there has been around 1000 confirmed flu cases - 100 in the last week alone - a significant increase on last year.

He advised aged care and health facilities to notify the public health unit as early as possible when multiple residents had flu symptoms, so they could act early to stop the spread of infection.

"The earlier the notification, the earlier we can offer support," Mr Gregory said.

"There's a number of processes - first we develop a line list of people who are sick, so we can pinpoint onset dates, symptoms and other pertinent information.

"This lets us track the outbreak and how it's developing and also lets us know when it's winding down and under control."

The public health unit works with facilities around infection control, which involves increasing hygiene practices, managing patient care and supporting staff.

"We cohort patients together - so sick patients are kept separate from healthy patients to help reduce transmission," Mr Gregory said.

"We often advise restricted access and ask people who are sick not to visit, and other visitors to take precautionary measures like wearing face masks."

Mr Gregory said many elderly people got the flu during winter as they were vulnerable.

"However they don't always die from the flu, because often they've got underlying co-morbidities and chronic illnesses," he said,

"We also know that during the winter months the mortality rate increases in this population because they are so vulnerable."