"(But) when it says Advance Australia Fair, it means advance the white people," the girl student told CNN-affiliate Nine News.
"And when it says 'we are young' it completely disregards the Indigenous Australians who were here before us for 50,000 years," said Nielsen.
According to the Courier-Mail newspaper, the school told the girl that she had to stand or leave the building. But when she refused to do either, she was handed a lunchtime detention for "blatant disrespect".
Nielsen was then told that she could not leave until she had signed a written apology and that she could be suspended.
Controversial right-wing senator Pauline Hanson said Australian schools were "brainwashing" children and called for Nielsen to be "taken out" of her school.
"It's about who we are as a nation, it's part of us ... Here we have a kid who's been brainwashed and I'll tell you what, I'd give her a kick up the backside," she said in a video posted on Facebook on Wednesday.
"This kid is headed down the wrong path and I blame the parents for encouraging this."
Former Australian Prime Minister Tony Abbott said the girl should "follow the rules".
Queensland Liberal National politician Jarrod Bleijie, the state's shadow minister for Education, called Nielsen a "brat". He said that the girl should be suspended if she continued to sit during the anthem.
The girl's father, Mark Nielsen, said his daughter was "very brave", adding he had met with the school's principal but no agreement had been reached.
In a statement, the Queensland Department of Education said the school had never suggested Nielsen would be expelled or suspended for refusing to stand during the anthem.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)