The Federal Government has conceded it might never be able to make the COVIDSafe coronavirus tracing app work on older phones as downloads surpass 5.1 million.

Deputy Chief Medical Officer Paul Kelly said the app worked on more than 90 per cent of smartphones in Australia but it was unclear if the remainder would ever be compatible with the app.

"One of the issues raised at the Australian Health Protection [Principal] Committee (AHPPC) yesterday is that there are some people with phones in Australia where the operating system is quite old and therefore this app is not compatible," he said.

"I can say that over 90 per cent of the market in Australia have operating systems which are compatible.

"So that means there are 10 per cent of phones which we may not be able to fix that issue."

The Federal Government launched COVIDSafe 10 days ago in a bid to make contact tracing quicker within the community.

The Prime Minister has repeatedly urged Australians to download the app and said more people doing so would give leaders greater confidence in easing coronavirus-induced restrictions.

National Cabinet will meet again on Friday, when it is expected to ease some of the restrictions.

Professor Kelly said that hitting 5 million downloads within 10 days made COVIDSafe the quickest app to reach that milestone in Australian history.

"I think that is a remarkable statement about how the Australian public has listened to our appeal for this particular piece of the puzzle," he said.

Professor Kelly said 62 people remained in hospital, with 26 new coronavirus cases reported in the last 24 hours.

That takes the total number of cases in Australia since the outbreak of the deadly pandemic to 6,875, including 97 deaths.

Senators assessing app's performance

The performance of COVIDSafe on iPhones was explored at a Senate committee hearing investigating Australia's coronavirus response.

Digital Transformation Agency (DTA) chief executive Randall Brugeaud said the contact tracing app's Bluetooth functionality might not always perform well on iPhone.

He said factors affecting performance included phone model, if the device was unlocked and if COVIDSafe was foregrounded, or visible on the screen.

That means COVIDSafe might not always be able to share or collect the IDs that represent people who have come into range of your smartphone — the function key to its benefit to contact tracing.

"The quality of the Bluetooth connectivity for phones that have the app installed running in the foreground is very good [but] it progressively deteriorates … you get to a point where the phone is locked and the app is running in the background," Mr Brugeaud said.

He said the DTA was working with Apple and Google to address the performance of the app.

Prime Minister Scott Morrison has repeatedly said he hoped at least 40 per cent of Australians would download the app to ensure it was effective.

Government officials, including the Chief Medical Officer, have been reluctant to put a figure on downloads in the last week. They have instead said every download helped.

Department of Health acting secretary Caroline Edwards told the Senate committee there was no uptake goal within her team, and she has not provided such advice to Government.

"Every single upload of this app and use of it is useful to help our health authorities to do their work," she said.

"I'll take one, I'll take 10 per cent, I'll take the 5 million we've got and if it gets to 40 per cent and beyond, well, then I'll be even more delighted," she said.

The Government estimates there are 16 million adult smartphone users that it is hoping to target.

Under repeated questioning, Ms Edwards said the Department had not conducted modelling on the link between uptake figures and the app's effectiveness.

She said COVIDSafe was intended only to "augment" the public health response, which includes traditional manual contact tracing.

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