Fair Work Australia is investigating whether it was legal for a contractor to be fired for showing support for the No campaign in the upcoming same-sex marriage survey.

Madlin Sims has gone public with her decision to terminate an entertainer from her children's party business in Canberra after they put a frame on their private Facebook profile picture that said "it's OK to vote no".

The performer at the centre of the controversy, identified only as Madeline, was an independent contractor with Ms Sims' business and not a direct employee.

Fair Work has told the ABC they are looking into the case.

"The Fair Work Ombudsman is aware of this matter and in order to form an assessment as to whether any workplace laws have been breached will be contacting the parties involved as part of its inquiries," a spokeswoman said.

Opposition Leader Bill Shorten said no-one should be fired over their choice to vote yes or no in the upcoming survey.

"I won't comment on that individual case," he said.

"But I want to make it very clear, people should not be dismissed from their employment for having different views about marriage equality in this country.

"People's job security shouldn't be threatened... no-one should risk losing their job because they are voting yes or no."

Government backbencher George Christensen has called on the Human Rights Commission to urgently investigate the matter.

Ms Sims said her brother raised a disagreement with Madeline over the use of the "it's OK to vote no" frame.

Key dates in SSM postal survey: August 24 — the final day to register with the AEC if you want to take part in the survey

September 12 — survey forms start being sent out

September 25 — all forms are expected to have been sent

October 27 — forms are strongly encouraged to be returned by this date

November 7 — the final deadline to return surveys

November 15 — results are released

In response Madeline said she believed same-sex marriage was wrong and claimed she was being attacked for her religious beliefs.

Ms Sims then took to Facebook herself, arguing that advocating against same-sex marriage was homophobic "hate speech", adding she did not want her business linked with the contractor's views.

"Just like I wouldn't want someone who is racist working with us ... I wasn't comfortable having someone who was so out and proud about being against equality," Ms Sims wrote.

Madeline spoke on ABC's Hack program last night and said putting the filter on her Facebook page was her only public action.

"I'm not afraid to stand up for my beliefs and being a Christian," Madeline said.

"Everyone else is putting up these Vote Yes filters, and there's one filter that says 'it's OK to vote No'. I thought I don't have to put this up but I don't have to stay silent."