Harper Nielsen claimed the song "Advance Australia Fair" ignored the nation's indigenous people.
"When it says 'we are young' it completely disregards the Indigenous Australians who were here before us," she told ABC news Australia.
Controversial right-wing senator Pauline Hanson labelled Harper a brat.
The schoolgirl was given detention last week for "blatant disrespect" over her failure to participate with classmates during a rendition of the song at Kenmore South State School in Brisbane.
Harper, whose parents said they were "proud" of her for showing "incredible bravery", said she felt it was time to "raise awareness and get people thinking".
"When it was originally written, Advance Australia Fair meant advance the white people of Australia," she said in an interview with ABC.
Harper has taken issue with the opening lyrics to the anthem, which read: "Australians all let us rejoice, for we are young and free."
But her protest has outraged some, including Ms Hanson, who posted a video response on Facebook describing Harper as "disgraceful".
"Here we have a kid that has been brainwashed and I tell you what, I would give her a kick up the backside," the 64-year-old said.
"We're talking about a child who has no idea," she says, adding: "This is divisive.
"I'm proud of the national anthem. It's about who we are as a nation."
Jarrod Bleijie, Queensland's shadow minister for education from the centre-right Liberal National Party, also criticised Harper's parents and called their daughter's actions a "silly protest".
A NINE-year-old girl has been threatened with suspension and given detention because she refuses to stand for the national anthem.
Others, such as Australian journalist and television host Georgie Gardner, praised Harper for her "strength and character".
"I do applaud her for considering the words of the national anthem, a lot of people just rattle it off and don't consider the meaning," she said.
On Twitter, users posted messages of support using the hashtag #HarperNielsen, calling the schoolgirl "Australia's most fantastic and brilliant brat" and "the hero Australia doesn't realise it needs".
Harper's move echoes the controversial kneeling protests of NFL players during the national anthem in the US, which began with quarterback Colin Kaepernick.
What is happening in Kaepernick row?
A spokesperson for the Queensland state education department said the school had offered Harper the choice of remaining outside the hall during the anthem or simply not singing.
In June, Australian states took steps towards the nation's first treaties with Aboriginal and Torres Strait Islander peoples.
Many indigenous Australians have cited a treaty or treaties as the best chance of bringing them substantive as well as symbolic recognition - the subject of a long-running national debate.
Australia is the only Commonwealth country that does not have a treaty with its indigenous populations.
BBC
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