Labor frontbencher Ed Husic says a new Liberal senator would have been disciplined if he shared social media posts attacking groups other than Muslims.
Jim Molan shared posts from extremist group Britain First - which has been linked to murders and terror attacks on Muslims - on his Facebook page last year and has refused to apologise for posting them.
Prime Minister Malcolm Turnbull strongly defended Senator Molan in Question Time on Tuesday.
But Mr Husic said the Liberal leader would not have done it if the posts had been misogynistic or homophobic.
"If anti-Semites had posted something terrible about my friends in the Jewish community, would we have received the same approach from the prime minister?" Mr Husic asked on Sky News on Wednesday.
"I doubt it, rightly so.
"I think he would not even hesitate to come out and say that was the wrong thing. He didn't in this case."
Thomas Mair yelled "Britain first" as he stabbed Labour MP Jo Cox to death in 2016, while a Britain First donor who drove his car at a curry van in 2017 told police he wanted to kill a Muslim.
Darren Osborne, who drove a van into a crowd of Muslims in London and killed one man, also became radicalised in part through reading Britain First content.
Senator Molan said it was "unwise" for him not to clean up his social media accounts before he was appointed to the Senate.
"But the reason I didn't do that of course is because I saw nothing wrong with (the re-posts)," he told Sky News.
Mr Turnbull on Tuesday defended Senator Molan as a "great Australian soldier".
The Greens used parliamentary privilege on Tuesday to question Senator Molan's military record, but he said he abided by the laws of armed conflict during the Iraq war.
