COVID-19 infection rates are significantly more widespread in Australia than has been reported, with as many as 20,000 carriers of the disease, modelling by the Actuaries Institute shows.

That was far higher than the active number of around 3000 officially reported at April 9.

"There are a lot more people with COVID-19 than there are confirmed cases,” said Douglas Isles, an actuary who has a master’s degree in mathematics from Cambridge University. “The confirmed active cases hugely under-report community infection."

Australian Department of Health data says that at April 9 330,000 Australians – more than 1% of the population – had been tested, with 6103 confirmed cases, 51 deaths and 2987 recoveries. The median age of death was 79 and the median age of those infected was 47,

"The key point is that the reported data is an order of magnitude less than the actual data,” Mr Isles said. "Those risks need to be better understood before authorities decide when and how to relax current restrictions – an issue looming large for policymakers.”

The Institute has established a COVID-19 working group with a core of 13 actuaries, which is supported by a further 50 actuaries. The group is advising business leaders, regulators and policymakers seeking to understand the impact of the virus on society and the economy.

The modelling tracks time lapses from the onset of the virus to recovery in both mild and severe cases. Mr Isles has used Australia-wide statistics, plus rate-of-population testing, number of positive results and recovery and fatality rates.

Using a constant carrier mortality rate and timeframe from infection to death allowed the model to infer new carriers at a point in time. Mr Isles applied a carrier mortality rate estimate of 0.5% to the data on Australian COVID-19 deaths.

That suggested there may have been around 400 new carriers each day in the week to March 12, and 800 each day in the week to March 19.

“This means it is likely there were around 10,000 carriers by March 19 and by April 9 more likely 20,000 carriers,” the study says.