Australian broadsheet newspapers The Sydney Morning Herald and The Age on Monday ditched their century-old format for tabloid in a major overhaul at ailing media giant Fairfax.

The move is part of the company's bid to shore up readership and advertising revenue in an increasingly digital landscape that has forced the closure of newspapers across the world.

Fairfax announced the sacking of 1,900 staff in June in a radical cost-saving move and plans to put the Herald, which has been a broadsheet since 1831, and Melbourne paper The Age, founded in 1854, behind a paywall this year.

Herald editor-in-chief Sean Aylmer said the move to a "compact" format on the weekday editions was in response to reader wishes, although the Saturday editions will remain in their traditional format.

"The research we've done shows that reading a compact newspaper is easier and preferable to reading a broadsheet," he said.

"So we've changed our shape. But nothing else is going to change. We are committed to quality journalism."

Fairfax, which has newspaper, radio and digital interests, is the main rival in Australia to News Limited, Rupert Murdoch's Australian empire, which is also suffering from the changed landscape.

In common with media companies worldwide, both stables are facing sliding print advertising and circulation revenues.

The Herald and Age websites have also been redesigned to be tablet-friendly, with a greater emphasis on picture stories and video.

But the move has been met with a mixed response. Of hundreds of reader comments left on the Herald website, most were negative.

"Five thumbs down. The layout is cluttered and confusing," said one reader.

"I go to a newspaper site for in-depth articles, not video. If I want TV-style news I will go to a TV news site," he added.

But others were more complimentary. "I am liking the new clean design. As always, it will take a bit of getting used to," another reader said.

Peter Fray, a former editor-in-chief of the Herald, said if the tabloid switch does not work, weekday print editions of Fairfax titles could disappear within five years.

"My gut feeling is that we may not see a printed Monday to Friday (edition) in say five years. Some people say it's much sooner than that, one to two years," he told ABC radio.

Former Age editor Mike Smith told the broadcaster the long-resisted change was a matter of survival.

"This is the most significant physical change to the Fairfax papers since they took ads off the front page," he said.

"It took a world war to do that, and it's taken a threat to their very existence to make them go tabloid."