Volunteer groups say they could be thousands of dollars out of pocket after a Federal Government frequency sell-off.

At the end of this year, it will be illegal to use a certain frequency which transmits wireless microphones.

The bandwidth range has been sold to mobile network operators to support the 4G broadband service, meaning community, church and school groups will have to upgrade their equipment.

The Woodville Uniting Church in Adelaide's west has been using its wireless microphone and PA system to deliver weekly services to its congregation for the past eight years, but will have to spend hundreds of dollars to upgrade the system because of changes to transmission frequencies.

Woodville Uniting Church clergyman Trevor Klar says he is not happy about the added expense.

"For me the issue is that for voluntary organisations there's a huge impost on their budgets, and raising money for any sort of voluntary organisation is always difficult," he said.

"For us, we've got two radio microphones, and if we were to replace like with like, we're talking about $1,300.

"Now, for a church, $1,300 is a lot of money to find from nowhere.

"I approached a couple of MPs, one from each political brand, and whilst their staff were very, very helpful, they directed me to state government and local government grants rather than anything from the Federal Government."

Woodville Uniting Church is just one of thousands of churches, community and school groups likely to be affected.

In 2010 the then Labor Federal Government decided to sell off frequencies between 694 and 820 MHz to mobile phone companies to support 4G services.

That means wireless microphone users will have to move to a different frequency band.

Anna Cheung from Australian Communications and Media Authority says they have been doing what they can to get the information out to community groups.

"Chances are if people purchased any equipment from a responsible supplier in the last 12 months or so, their equipment would already comply," Ms Cheung said.

"So what we're telling people first off is just check the equipment because chances are you should be sorted already."

Consumer anger over replacement cost

But callers to ABC local radio in Adelaide say the change is news to them.

"We're going to have to replace nine that I can see. I just checked one this morning and it's within the band," one caller said.

"I hadn't heard anything about this change of signal, so I'm a bit worried I'm going to have to go and find a budget now, and I've used my budget up this year."

Mark Gregory from the school of electrical and computer engineering at RMIT says changes to how frequencies are used happen all the time and the Government has weighed up the cost and benefits of making that change.

"One of the benefits of the particular band that was being used is that it has a good range. It's a very useful range around buildings and in built-up areas," he said.

"So it was particularly good for wireless microphones. The problem, of course, is that it's also good for things like mobile cellular networks.

"Currently Australia is experiencing huge demand for more and more 4G resources.

"What it means really is that because it's going to be allocated to a very important system like 4G, is that people with these older devices will find themselves causing interference with the 4G if they continue to use them, and that means that people will be looking for them.

"It could [cost] anything from a couple of hundred dollars to several thousand dollars.

"Some of these wireless microphones and speaker systems are not cheap. I can understand why these groups would be very concerned by this."

It is estimated the cost of replacing equipment nationally will be more than $200 million.