High-profile Australian journalists and large media organisations have gone on trial charged with breaching a gag order in reporting Cardinal George Pell's sex abuse convictions in 2018, which have since been overturned.

A total of 18 individual journalists, editors and broadcasters face potential prison sentences, and 12 organisations face fines if they are found guilty in the Victoria state Supreme Court of breaching a judge's suppression order on Cardinal Pell's case. All have pleaded not guilty.

Such suppression orders are common in the Australian and British judicial systems. But the enormous international interest in an Australian criminal trial with global ramifications highlighted the difficulty in enforcing the orders in the digital age.

Cardinal Pell was convicted on 11th December, 2018, of sexually abusing two choirboys in a Melbourne cathedral when he was the city's archbishop in the late 1990s.

The trial of Pope Francis's former finance minister and the most senior Catholic to be charged with child sex abuse had not been reported in the Australian news media because of a suppression order that forbade publication of details in any format that could be accessed from within the country.

Details were suppressed to prevent prejudicing jurors in a second child abuse trial that Cardinal Pell was to face three months later.

That second trial was cancelled due to a lack of evidence and Australia's High Court in April overturned all convictions after Cardinal Pell had spent 13 months in prison.

In opening her case, prosecutor Lisa De Ferrari said the morning after Cardinal Pell's convictions, Australians could read about it on overseas websites, with US-based The Daily Beast among the first to break the news.

One online story published by Melbourne's Herald Sun that referenced overseas reporting led to the newspaper's owner and staff being charged.

No foreign news organisation has been charged with breaching the suppression order.

The media defendants have not said what their defence will be. Ms De Ferrari said some of those charged had said they were not aware a suppression order existed.

Justice John Dixon is hearing the trial without a jury and via video links due to pandemic restrictions. The hearing is expected to take two to three weeks.