As many as 500,000 Australians could have been infected with coronavirus and not necessarily know it, according to New South Wales chief health officer Kerry Chant.
It's on the higher end of figures Chant said are being studied to determine the number of people who have been exposed to the coronavirus, which an early estimate puts at between 1 and 2 per cent of the total population in Australia.
Public health authorities have been conducting serological testing to identify antibodies in people's blood in the hopes of further understanding the virus' presence in Australia.
"That study is being undertaken at the moment and will be reported in due course," Chant said.
"But it's an important component of our surveillance plan to actually understand how many people at particular points have been exposed," she added.
University of South Australia epidemiologist Professor Adrian Esterman told news.com.au "it would be a surprise" if that many people had been infected, and it would mean many might have never known they had coronavirus.
"It would mean that there are an awful lot of people who have been infected, had few if any symptoms, and therefore never got on the radar," Professor Esterman said.
"If they did pass it on, then the newly infected people might be in the same situation."
"The ones we are finding from PCR testing (that looks for traces of the virus in the swabs), presumably are far more likely to be symptomatic and end up in hospital," he added.
While Covid-19 remains a sometimes deadly and highly infectious disease, Chant said the figures reinforce what we already know: some people are better at dealing with the virus than others.
For some, "Covid is a mild disease," Chant said, so while many more people than we know about may be infected, "not everyone will present for care or be diagnosed".
That doesn't mean they can't still be infected and spread the disease to more vulnerable people without realising.
Researchers at the National Centre for Immunisation Research and Surveillance and the Kirby Institute at the University of NSW have been leading the study and had almost finished a survey of Sydney, according to the Australian Financial Review.
Blood samples are also being collected across the country for a national study.
While 500,000 Australians sounds like a lot, Chant said the numbers show how effective the country has been at limiting the spread so far.
She compared it to hard-hit New York City where 20 per cent of the population (about 1.7 million people) are estimated to have been exposed to the virus.
It comes as NSW deals with small outbreaks. NSW Health issued a warning to churchgoers in southwest Sydney after a woman attended several services, including a funeral, while infected last week.
The outbreak in Victoria continues to grow. There were eight deaths recorded on Friday, making it the state's deadliest day since the pandemic began.