SYDNEY - Australian Prime Minister Scott Morrison wants all residents to use a coronavirus-tracing app modelled on Singapore's, saying this could help to further drive down the country's already low infection rates.

But the proposal has been controversial, with critics saying it is unnecessary and raises privacy concerns.

Mr Morrison said he wants a minimum 40 per cent usage rate of the app across the country, saying this will allow the current lockdown to be eased more quickly. He said the app will be voluntary, though he has strongly urged people to use it.

"It will be optional, but we need to get a big take-up," he told 3AW radio.

"If people take up this app, that means we have greater confidence that if someone gets the coronavirus, we can more quickly trace down their contacts... If we can't do that… then we'll have to keep the restrictions in for longer."

Australia had 6,652 confirmed cases of Covid-19 and 74 deaths as of Wednesday (April 22). But daily case numbers have been steadily dropping, raising hopes that restrictions such as bans on large gatherings and most non-essential businesses will be lifted.

On Wednesday, the country recorded 12 new cases, down from as many as 460 on March 28.

Australia's reduced infection rates have been credited to widespread testing, a ban on international visitors, 14-day quarantines for Australians arriving from abroad, and strict social distancing rules.

But the government says the app will be the necessary next step to ensure cases keep dropping.

The app, which is still being developed, will not allow location tracking or surveillance. Instead, it will use Bluetooth to connect with the app if it has been downloaded to another person's phone if the other phone is within 1.5m for 15 minutes.

If a person contracts Covid-19, health officials will be provided with details of all those whose phones had been close to the infected person within the previous 21 days and then contact those people.

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The app, based on Singapore's TraceTogether, is due to be made available for free within the coming weeks.

So far, Australians have been largely compliant with strict social distancing measures but the app has attracted a wide range of critics, including some MPs.

They say the app hands excessive control to the government of private information. Some have urged the government to guarantee that the information will be used for health purposes only and not by police or security agencies.

An expert on information technology law, Professor Graham Greenleaf, from the University of New South Wales, said the federal and state governments should legislate to guarantee that the app will be voluntary and that access to it and its data will end when the health emergency is over.

"Laws should guarantee that these are emergency measures, not permanent ones," he wrote in an article for the university.

"The success of Australia's coronavirus responses has been substantially because of public trust... Governments now need to… (ensure) that its proposed contact tracing app is necessary and proportionate."

A prominent member of the ruling Liberal-National Coalition, Mr Barnaby Joyce, said he was concerned about privacy and did not plan to use the app.

"I treasure the government knowing as little about me as possible," he told The Sun Herald.

The Labor opposition party has expressed support for the app but urged the government to provide more details about privacy protections and oversight.

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The Government Services Minister, Mr Stuart Robert, tried to boost confidence in the app this week, saying the source code and an assessment of the privacy impact would be released. He said the app and all data that had been collected would be deleted when the pandemic ends.

"We don't care where you are or what you're doing," he told ABC News.

"It will allow us to get back to work quicker... We need to do this for the country."

The app is due to be discussed by the national Cabinet of federal and state leaders on Friday.