Justice Brian Leveson, who led the inquiry into Britain's phone-hacking scandal, said Friday the Internet's growth had complicated debate about mass media privacy, with no easy solution.
Leveson, who last month called for an independent media regulator in Britain, told a privacy symposium in Sydney that there was "an element of mob rule" in identifying people online, and said it was a difficult area to police.
"To name and shame people by broadcasting their behaviour (online), there is a danger of real harm being done, and in some cases harm which is both permanent and disproportionate," he said.
"There is not only danger of trial by Twitter, but also of an unending punishment, and no prospect of rehabilitation, via Google."
He said too many people were unaware that images and data uploaded online could be stored forever.
"Children and the young do not appreciate that uploading a compromising photograph for a laugh can have consequences for the long-term future," he said.
"Because once the photograph is in the public domain, it can be found, copied and reproduced, all, again, at the click of the mouse."
The role of the Internet and social media in Australia has come under the microscope of the government, but Communications Minister Stephen Conroy has admitted it is difficult to do much about US companies operating under US law.
"In the past there's been a whole range of pages, not just on Facebook but on other sites where people have made complaints," he said earlier this year.
"We've gone to the courts, the courts have issued notices, the police have gone to enforce them over in the US and in the past we've got nowhere."
Leveson said he had no easy solutions, admitting it would be difficult to restrict the Internet's power given there had been a "historical failure to develop limitations on incursions into privacy by the media".
"It might reasonably be said that it is difficult to assume that any such limitations might evolve insofar as the Internet is concerned," he said.
Leveson led an eight-month inquiry into the media sparked by the phone-hacking scandal at Rupert Murdoch's News of the World tabloid.
In his report, he said the British newspaper industry had for decades "wreaked havoc with the lives of innocent people" and ignored the codes that it had itself set up.