The makers of the Covidsafe app have had to overhaul the algorithm for determining close contacts to improve the functionality of the app on iPhones, Guardian Australia has learned, as new data suggests it is working only 27% of the time on some Apple mobiles.

Since launching in April, the Covidsafe app has consistently struggled to work on iPhones when the app is running in the background, or the screen is locked, due to issues with ensuring the bluetooth beacons or “digital handshakes” are sent out to nearby devices of potential close contacts.

In June, a table provided to the Senate by the agency responsible for the development of the app, the Digital Transformation Agency, detailed test data from various devices running the app. It showed the app sending out and receiving beacons as infrequently as one in four times on the iPhone.

Under freedom of information laws, Guardian Australia requested a copy of the underlying data used to produce the table provided to the Senate committee, and in response, DTA provided not only the underlying data, but the most recent test data and background information on changes made to the app.

As of 1 August, the older iPhone 6 and 6s models were still only communicating with each other or iPhone X models between 27% and 40% of the time when both screens were locked. It is a substantial improvement, however, from the late April test results that showed the app working as infrequently as 7% of the time.

In the most ideal scenario where both apps are active and the phones unlocked, the iPhones communicated between 73% and 93% of the time as of 1 August.

For communication between iPhones and Android devices, the data showed the app was working as often as 100% of the time when both phones were active, but as infrequently as 40% of the time when they were locked.

In a background document provided in the FOI response, the DTA said the iPhone’s performance now was close to being rated “good”, but said changes made “across the whole Covidsafe ecosystem”, in particular the algorithm used by health officials in determining close contacts based on the bluetooth data, had allowed it to work better on iPhones.

“If any digital handshakes are not received, for reasons such as bluetooth performance or environmental factors, the health portal [used by state health officials] is able to overcome any lost handshakes following improvements to its filtering algorithm,” DTA said.

“The algorithm now determines a pattern of behaviour over a 15-minute window, rather than requiring a full set of 15 consecutive handshakes.”

The app has had success in New South Wales, where 14 close contacts not identified by manual tracing were identified. Although none of those contacts tested positive, it did allow health officials to find a previously unknown infection date at Mounties club, and through notifying more than 500 patrons who were there on that date, two more confirmed cases of Covid-19 were discovered.

At a hearing of the Senate select committee on the Covid-19 response last week, Caroline Edwards, the health department associate secretary, confirmed the app had been less useful in Victoria due to the way that state’s contact tracing system had been operating.

“We have been offering and delivering training to officers in Victoria, and we are now getting some additional activity for them to use the app to a higher degree,” she said.

“But to date, in the context of the large amount of community transmission and the really difficult circumstances they’ve found themselves in, they haven’t used it yet to the extent we would like.”

At one point early on, Victoria ceased using the app in contact tracing entirely, but has since resumed albeit in a limited way given stage four restrictions significantly reduce the chances of people encountering people they don’t know for 15 minutes.

The privacy rules around the handling of the Covidsafe app data mean the federal government has no oversight over how the app is working in contact tracing, except for what the state departments report back.

NSW is now leading a process to provide regular updates to the federal government on the utility of the Covidsafe app.