Prime Minister Scott Morrison has ruled out forcing Australians to download a coronavirus tracing app, one day after the Deputy Chief Medical Officer left the door open to making it mandatory.

Key points: On Friday, Scott Morrison said it was his 'very strong preference' that Australians download the tracing app voluntarily

On Friday, Scott Morrison said it was his 'very strong preference' that Australians download the tracing app voluntarily The Federal Government needs at least 40 per cent of people to sign up for the app to work effectively

The Federal Government needs at least 40 per cent of people to sign up for the app to work effectively The Prime Minister is asking for the support and cooperation of Australians

The Government is developing an app to bolster its ability to trace the contacts of infected Australians, but has warned it will not be effective unless at least 40 per cent of Australians download it.

The app would use data from people's phones to allow health authorities to trace people who had been in close contact with confirmed COVID-19 cases.

Yesterday, Deputy Chief Medical Officer Paul Kelly said the Government would "start with voluntary" downloads of the app, before determining whether more action was needed.

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"I've always been a believer in the Australian people making the right decision," he said.

"As I've said, this is an add-on to what we have in terms of contact tracing and case finding, so I think we need to make the case for an app.

"I think we start with voluntary, and see how that goes."

But this morning Mr Morrison tweeted that the app would not be made mandatory.

"The app we are working on to help our health workers trace people who have been in contact with coronavirus will not be mandatory," he said.

"We will be seeking the cooperation and support of Australians to download the app to help our health workers, to protect our community and help get our economy going again."

The tweet clarifies comments from Mr Morrison yesterday, in which he said it was his "very strong preference" that Australians download the app voluntarily.

Senior cyber experts looking at app security

Minister for Government Services Stuart Robert sought to again assure people that the privacy and security of the app would be watertight.

"There is no geolocation, there is no surveillance, there is no tracking," he said.

"The app simply connects with another app if those two phones are within 1.5 metres for 15 minutes.

"It simply swaps phone numbers and names."

He said the Government had enlisted the help of the Australian Signals Directorate and the Australian Cyber Security Centre, as well as other industry partners, to check the veracity of the security measures in place.

Mr Robert also said the Government would be publishing the source code for the app for people to view and comment on, as well as the privacy impact assessment when it was completed.

"We want to be as transparent as we possibly can," he said.

"It is a big Team Australia moment.

"When this app is released in the next week or two, we really need every Australian to download it and to run it."

The Government's app will be modelled off one used in Singapore, where only 20 per cent of people have downloaded it.

The app is called TraceTogether and uses Bluetooth to create a record of other nearby phones that also have the app.