A union official who threatened to "go to war" with an Adelaide building site manager has been ordered to pay Federal Court legal costs for contempt.

CFMEU organiser Jim O’Connor was in May found to have threatened industrial action with management of the Royal Adelaide Hospital construction if a union member wasn’t given a job.

He told a contractor at the site: “You don’t want to go to war with us.”

The comment was found to be in contempt of a Federal Court order placed on Mr O’Connor for an earlier incident at another Adelaide worksite.  Justice John Mansfield on Friday ordered Mr O’Connor to pay a $12,000 fine and legal costs.

CFMEU South Australia secretary Aaron Cartledge said the outcome was disappointing and the union would consider appealing.

“We will support Jimmy O’Connor,” he said outside Adelaide’s Federal Court.

“Jimmy works very hard at looking after our members, looking after their lives on the job, protecting wages and conditions.”

Mr Cartledge said the decision wouldn’t change the union’s approach to supporting members.

“We’ll continue to defend our members on building sites, on mine sites and on manufacturing sites,” he said.

“The issue’s too important to walk away. Their lives are at risk every day, and they’re at risk of being exploited every day.”