More than 10 per cent of COVID-19 cases in Australia have mysterious origins. Experts say a good proportion of them could be ‘sleepers’ that will lead to further infection.

We may have to tolerate a low level of infection going forward with normal human endeavours business, schooling and the like but I dont think they are too big a risk to us,” he said.
Although they could initiate chains of transmission, Professor Booys thinks Australia could still get away with opening the country. 
Weve learnt how to do physical distancing and it can be an important compensator for the sleeper cases. We need to keep it up but we dont need to be so rigid. Basically, we need to test to see what we can and what we cant get away with.
Health authorities do not know the origin of almost 700 cases, with the majority of these cases from NSW (439). Victoria has 137 cases of unknown origin, followed by Queensland (61), Western Australia (22), South Australia (13), the ACT (7), Tasmania (4) and the Northern Territory (1).
The percentage of unknown origin cases has increased from 7.7 per cent at the start of April to more than 10 per cent by April 26.
Despite the uncertainty caused by these cases, the overall growth rate of new COVID-19 infections was just 0.1 per cent on Monday.
There were only six new cases, three in Queensland, two in NSW and one in Victoria, bringing the national total to 6718.
South Australia registered its fifth day of no new cases, while Western Australia and the ACT both reported a second day of no new cases, as of 5.30pm AEST. No new deaths were reported, leaving the total at 83.
Dr Gnana K Bharathy from the School of Information Systems and Modelling at UTS, said COVID-19 is a slightly unusual pandemic with its long asymptomatic incubation period, when the infected may be shedding virus.
Traditional models are based on symptomatic transmissions, says Dr Bharathy.  
He said traditional models are based on symptomatic transmissions. But there are estimates that as many as 80 per cent of people who are infected are asymptomatic. They too may be carriers.
The actual transmission and prevalence might be higher, but goes unnoticed until the infected is tested or starts to show symptoms or seriousness. So, testing is vital.
Fortunately for us, with wide-spread testing, we still have low positive test results.
He said the matter of immunity is very important. It is not known whether post infection, immunity is powerful enough to resist reinfection, how long it lasts and how it would respond to different strains of the virus.
Dr Bharathy is part of a team that is building a model that takes into account asymptomatic infection and loss of immunity, which makes recovered populations available for re-infection.
He believes the virus will stay-on for a while. Cases of unknown origin, asymptomatic cases and unknowns about immunity from diagnosed cases will form a reservoir for spread.
Relaxing lockdowns will need to be managed through wide-spread testing and continued advocacy of social distancing and hygiene practices.
Widespread testing would also inform us about re-infection likelihood.