The NSW Industrial Relations Commission (IRC) Justice Wayne Haylen on Thursday morning fined the union 6,000 AU dollars (6,068 U.S. dollars) for its strike on Sept. 2011, the Australian Associated Press (AAP) reported.

As many as 67,000 teachers walked off the job on that day over a 2.5 percent cap on public sector wages.

The 24-hour action defied an 11th hour ruling from the IRC.

"The court is unable to conclude that these breaches are of the same order of seriousness as the previous two breaches by the federation," Haylen said in his judgment.

"This industrial action involved protest rallies against government policy not open to arbitration."

The union has been ordered to pay the fine within 28 days.