“There is not only a danger of trial by Twitter, but also of an unending punishment, and no prospect of rehabilitation, via Google,” warned Lord Justice Brian Leveson in Sydney this morning.

“Instances of naming and shaming are relatively commonplace. It takes but a minute to record someone doing something in a public place and to upload it up to the internet. Once on the internet the episode, the behaviour, is there for the world to see and is there permanently at the click of a mouse.”

The resulting harm can be “both permanent and disproportionate”.

“Children and the young do not appreciate that uploading a compromising photograph for a laugh can have consequences for the long term future because once the photograph is in the public domain, it can be found, copied and reproduced,” he said.

Leveson was giving the keynote address to today’s “Privacy in the 21st Century” symposium, organised by the Communications Law Centre at the University of Technology Sydney. He set his warning in an historical context.

The gossip-filled “penny press” of 1830s England, the tabloids of their day, were wildly popular and seen as a threat to members of society. So were the emerging technologies such as the telephone, the wiretap and photography. But in time the excesses of the new media operations were reined in by new laws and new social norms.

“In many ways we’ve been here before,” Leveson said. But he also also saw key differences when it came to the internet.

While newspapers and other traditional news media might push the boundaries when it comes to invading privacy, they’re generally held in check by commercial pressures as much as the law, said Leveson. Not so for the internet.

“With some exceptions for bloggers who carry advertising and tweeters who are sponsored, online bloggers or tweeters are not subject to the financial incentives which affect the print media, and which would persuade the press not to overstep society’s values and ethical standards. Equally, there is a view that blogging or tweeting is publication without responsibility or accountability and that, in this sense, the internet is beyond the reach of the law,” he said.

Leveson pointed to the 2011 “super-injunction spring” in England and Wales, when interim injunctions were granted prohibiting the disclosure of both the substantive content of the injunction and the very existence of the injunction itself. Traditional media complied. Bloggers and tweeters didn’t.

“Rumour and speculation regarding the number of such injunctions was rife. Twitter in particular and various blogs circulated and discussed names of individuals who were said to have obtained such injunctions. Names of various celebrities swirled around the internet,” he said.

“Bloggers rejoice in placing their servers outside the jurisdiction where different laws apply.”

But internet users are not actually beyond the reach of the law, Leveson said, citing as examples the prosecution of individuals involved in the UK’s autumn 2011 riots, and Twitter users being sued for defamation for wrongly identifying someone as a p-edophile.

“Given that the internet is not entirely out of the law’s reach it is likely in time that, as with the media in the 19th century, it will start to have an effect in individuals’ behaviour. It will start to modulate behaviour, and curb its wilder excesses. Time and proper application of the law will play the same role for the internet as it has done in all other areas of our lives. It will shape our behaviour and help to reinforce social norms,” Leveson said.

“This is not to say that social norms may not change over time. Our view of what is, for instance, private information may change. Our view of privacy may change. I imagine though that individuals will always seek to preserve some degree of privacy and will seek the law’s protection to protect it.”

Leveson did not comment on his recent report on media ethics to the UK government. “I treat the report as a judgement, and a judgement must speak for itself,” he said.

The symposium continues this afternoon.