The frequencies used by the stations will be shuttered by December 2015, Communications Minister Malcolm Turnbull announced on Wednesday.
They may then be sold to telcos eager to boost their mobile internet services at an auction that could potentially net the government billions of dollars.
Mr Turnbull said viewership was too small to justify the use of the valuable spectrum for community TV, urging stations to stream online instead.
"It will deliver wider audiences, at less cost, on a wider range of devices and the ability to do more than linear broadcasting," he said in a speech at a communications convention in Sydney.
But operations manager at Melbourne's Channel 31, Matt Field, said the transition would likely strip community TV stations of their advertising income from local small businesses.
"It's going to be very difficult to convince those supporters of the station to migrate with us to internet-only platforms," he said.
"It's very likely that this will spell the end for community TV in Australia for good."
He said the 15-month deadline was too soon for Channel 31, which employs 45 staff and reaches one million viewers a month, and has an annual revenue of $2.6 million.
"It's a pretty devastating decision," he said, adding the industry had not been consulted.
The spectrum is known as the "sixth channel" in addition to the five given to commercial broadcasters, SBS and the ABC.
It would be prized by telcos because of its low frequency, which allows it to carry internet transmissions further and penetrate buildings better than higher frequency space.
An auction of similar 700MHz spectrum, which was nabbed from TV stations when they went digital, scored the government about $2 billion from Telstra and Optus, which will use it for 4G services.
