Business groups have rounded on new ACTU secretary Sally McManus' comments condoning breaking "unjust" laws, calling them "grossly irresponsible" and a signal business will struggle to work constructively with the peak union body.

Ms McManus has doubled down on the comments made on ABC's 7.30 program on Wednesday that she did not have a problem with unions taking illegal industrial action when the laws were wrong, arguing that was how Medicare and working conditions in Australia had been won.

Business Council of Australia chief executive Jennifer Westacott said Ms McManus' comments could have dangerous consequences by "signalling to individuals around the country that it's OK to take the law into their own hands".

"What kind of society will we have if everyone can simply choose to ignore any laws they don't like?" Ms Westacott said in a statement.