It's the first time the federal government's new workplace laws have been used to issue fines over strike action.
The head of the Australian Council of Trade Unions, Mr Greg Combet, says the workers and their families are under intense pressure.
"They're deeply concerned. A number of them are very concerned that if a charge is ultimately levied against them, that they might have to sell their home. It puts their household assets in jeopardy, it's going to obviously have a severe financial impact if they have to front up to fines like this," he says.
Mr Combet says the union movement has set up a fund to cover legal costs.