The ABC will switch on Australia's first free-to-air digital 24-hour news channel tonight.

ABC News 24 will start broadcasting across the country at 7:30pm (AEST).

The launch special will be broadcast on ABC 1 as well as digital channel 24.

It will feature coverage of the news of the day from ABC News 24's broadcast centre at Ultimo in Sydney.

ABC News Director Kate Torney said it was an important day for ABC News.

"This is a real symbol of how important news is as part of the service that we deliver to Australian audiences," she said.

Ms Torney said the service had been funded through production savings, not new money.

She said the needs of the broadcaster's audience had changed and ABC News 24 would feature existing programs as well as new ones and rolling coverage of breaking stories.

"Audiences now expect to see news when it happens and they also expect to be able to tap into news when it suits them," she said.

"It's really important to deliver scheduled news to audiences on ABC 1 through the Midday Report and 7:00pm bulletins.

"So bulletins remain our flagship bulletins."

Critics argue the channel duplicates services that already exist and is a waste of taxpayers' money.

But Ms Torney says news is critical to the service the ABC offers and the 24-hour news channel is a chance for audiences to see news as it happens.

"This service is designed to ensure we are remaining relevant to modern audiences and we are giving audiences the services they expect at this time," she said.

"Our motivation has absolutely been to deliver what audiences need and want from the ABC and to deliver value for money to Australians."

ABC News 24 will vie with the only other Australian around-the-clock news channel, Sky News, which is broadcast on pay-TV provider Foxtel.

Foxtel chief executive Kim Williams has told Radio National Breakfast he relishes the competition.

"We wish the ABC well in its news endeavours," he said.

"Sky News will certainly provide an extremely competitive proposition."