The reception in the region had just been improved at the end of December after the new Mount Baranduda tower was switched on, but quickly became even worse than before.

Some people reported having to step outside their homes to use phones, and the coverage became particularly bad in the early mornings and dinner time.

Others thought their own modems had broken.

Telstra quickly identified the issue was being caused by the illegal use of mobile repeater device, detected near the Beechworth mobile base station.

But it was only last week that the device was finally located and shut down, returning mobile phone reception to normal.

Mobile repeater devices are designed to replicate and improve a user’s mobile signal, but can interfere with mobile coverage in surrounding areas when used without a licence or the right technical specifications.

Telstra area general manager Loretta Willaton said people may not realise what they are doing is illegal or interfering with others’ mobile coverage.

“The illegal use of these devices can cause significant mobile coverage issues for local communities and has even been known to knock out coverage to an entire area,” she said.

“While some people in an area might not notice a change, often those towards the edge of a coverage area will notice they no longer have any coverage, which can impact their ability to make triple zero emergency calls.

“Obviously there could be very serious consequences and anyone illegally using mobile phone repeaters need to be aware that their actions in breaking the law carries severe penalties.”

The unlicensed use of a mobile phone repeater can carry a two-year jail term per offence for individuals, or up to a $255,000 fine per offence for body corporates.

Ms Willaton said Telstra had been working with local authorities to identify the source and location of the interference, but it is not known if any charges have been laid yet against the offender.