The High Court will soon decide whether a worker can be sacked for calling his colleagues "scabs".
The case of a BHP Coal worker fired in 2012 for waving a sign that read, "SCABS - no principles - no guts", outside his workplace was heard in the High Court in Brisbane on Wednesday.
Machinery operator and union official Henk Doevendans was protesting against workers entering the Saraji coal mine near Dysart in central Queensland during protected strike action.
BHP Coal was ordered to re-hire Mr Doevendans after the Construction, Forestry, Mining and Energy Union successfully appealed his dismissal in the Federal Court.
However the order was overruled last December after an appeal to the full Federal Court.
On Wednesday counsel for the CFMEU said Mr Doevendans' behaviour was part of protected industrial action and he couldn't be sacked under the Fair Work Act.
"He was a participant in a lawful industrial activity organised by the union," Herman Borenstein QC told the court.
Counsel for BHP Coal argued Mr Doevendans behaviour wasn't protected, comparing it to being sexist or racist to a colleague at work.
The five High Court judges reserved their decision.