Nicholas Orde Jamison Abel along with Lesley Michelle Mosbey and Sarah "Daisy" Edwards fronted the ACT Magistrates Court on Thursday when they all pleaded guilty to defacing public property.

The court heard the trio splashed red paint at one of the department of agriculture's signs where they assembled before conducting a protest along Kings Avenue Bridge on Wednesday.

The 77-year-old Abel, a retired environmental scientist who had worked at the ANU and CSIRO, told the court the last time he committed an offence was in 1988 in the UK when he cut a fence to a US nuclear missile base to protest against war.

"I'll continue to break the law if that's what it takes to avert a major catastrophe," he said.

"You're dealing with a government that's sold its soul to the fossil fuel industry.

"You no longer need more scientific information. It's up to citizens now. Yesterday, I was acting as a citizen and I committed a minor crime to avert a major crime just as I did in 1988."

Abel said the future was "unimaginably awful" as the world was now at a tipping point.

He said he took part in the defacing and protest for his seven grandchildren, other people and animals.

Fellow protester Lesley Michelle Mosbey, 59, broke down in tears and said she took part because "I have exhausted all efforts".

"We've been told by scientists that we have three to four years to make significant changes or we'll be heading for hell on Earth," she said.

"I don't have any resources, I don't have any power or influence or wealth in the world.

"All I have is this body and my freedom and I'm going to keep throwing them at the criminal justice system because it's the only thing I know how to do that's going to attract attention and wake up our country."

Mosbey, whom the court heard was medically retired after working 20 years in drug and alcohol counselling, said "the time for niceties and conventions are gone" and that she would continue to protest.

Special Magistrate Jane Campbell dismissed Abel's charge, while Chief Magistrate Lorraine Walker convicted and fined Mosbey $200.

Sarah "Daisy" Edwards, 48, was convicted and fined $100.

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Ms Campbell said it was inappropriate for her to express her views about climate issues in the courtroom.

She said she accepted that both Abel and Edwards had honourable motivation in their actions and sentenced them based on the seriousness of the offending and other sentencing principles.

Ms Walker said she accepted that Mosbey was genuine in her pleas for climate action but that her defacing public property was a crime.

The court heard Mosbey is also before the court on other charges related to a previous protest.

Following the dismissal in Abel's case, supporters in court gave him a round of applause.

Two other people alleged to have committed crimes during the protest also fronted court. However, their charges still require the Commonwealth Director of Public Prosecutions' consent to prosecute.

They are scheduled to appear in court again on August 12

All defendants were self-represented in court.

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