Government officials have conceded the performance of their coronavirus contact tracing software is "highly variable" but insist it will still help health authorities.
More than 5.1 million people have downloaded and registered on the COVIDSafe mobile phone app since it was released.
Over the past week, the tech community has examined the app's code and have identified a number of functional and privacy issues.
Digital Transformation Agency head Randall Brugeaud told senators on Wednesday the app was being "constantly improved".
But he said suggestions the app did not work on locked iPhones was not his agency's experience.
The Australian government's new voluntary coronavirus tracing app. Source: AAP
Rather, performance "progressively deteriorates" when the phone is locked and the app is running in the background.
"I cannot provide a view that the app will work 100 per cent of the time with all handsets where the devices are locked," he told a Senate committee hearing.
"There will be circumstances where the app does not capture a Bluetooth handshake."
These issues only related to iPhones.
But software developer Jim Mussared has discovered problems with the app on Android devices that could potentially leave them open to being tracked for days.
Mr Mussared still recommends most people with Android phones should install the software.
"But if you're in any way in a vulnerable situation for which long-term, multi-day device tracking could be a major threat, do not install the app," he said.
His advice is similar to that given by WESNET, an organisation that offers technology safety advice for domestic violence victims.
It says those whose abusers have sophisticated technical abilities should think about their specific personal circumstances before downloading the app, and suggests if they do install it, they consider leaving their phone behind during any meetings they might want to keep secret.
Mr Brugeaud said his agency was engaging with the tech community on a range of issues relating to the app, but its ability to reach out was limited.
The health department expects to finalise agreements by the end of this week to allow the states and territories to access the contact tracing data.
The prime minister and cabinet colleagues have repeatedly said they want 40 per cent of Australia's 16 million smartphone owners to download COVIDSafe.
But acting department secretary Caroline Edwards said every single user was helpful.
"I'll take one, I'll take 10 per cent, I'll take the five million we've got ... and if it gets to 40 per cent and beyond, well, I'll be even more delighted," she said.
Labor's Murray Watt expressed concern about the link between app downloads and lifting restrictions on businesses.
"Here we are two days before that national cabinet meeting with an app that doesn't work all the time, that doesn't necessarily work when iPhones are locked, it doesn't necessarily work when Bluetooth signals are low, it doesn't necessarily work when it's in the background, only half the number of people have downloaded it that the prime minister said was required," he said.
"I'm just questioning whether we can have any confidence that this app is where it needs to be for Friday's meeting when we're all hoping that we're going to see some restrictions lifted."
People in Australia must stay at least 1.5 metres away from others. Check your state’s restrictions on gathering limits.
Testing for coronavirus is now widely available across Australia. If you are experiencing cold or flu symptoms, arrange a test by calling your doctor or contact the Coronavirus Health Information Hotline on 1800 020 080.
The federal government's coronavirus tracing app COVIDSafe is available for download from your phone's app store.
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