A US student said she was left stunned after her professor failed her on an assignment because she "incorrectly" identified Australia as a country.

Idaho stay-at-home mum Ashley Arnold, 27, said her professor failed her on the piece of assessment on February 1, claiming "Australia is a continent; not a country", Buzzfeed News reports .

The assignment, completed as part of an online degree at Southern New Hampshire University, involved comparing social norms between US and another country.

“At first I thought it was a joke – this can’t be real," Ms Arnold said.

“With (the professor's) education levels, her expertise, who wouldn’t know Australia is a country?

"If she’s hesitating or questioning that, why wouldn’t she just Google that herself?”

Idaho stay-at-home mum Ashley Arnold, 27, said her professor failed her on the piece of assessment on February 1. (Ashley Arnold)

Ms Arnold said she attempted to dispute the grade, only for the professor to send her a patronising response denying Australia was a country.

"Research is like dominoes, if you accidentally knock over one piece the entire set will also fall," she wrote.

"Australia is a continent; it is not a country.

"That error made it nearly impossible for you to accurately complete your week two research outline correctly."

Ms Arnold said she persisted, providing the professor with references validating her claim.

The other woman responded by thanking her and promising to "do some independent research on the continent/country issue" and to then review Ms Arnold's paper.

“With (the professor's) education levels, her expertise, who wouldn’t know Australia is a country?" (Ashley Arnold)

Ms Arnold said she was so incensed by the response she lodged a report with the university.

"I’m not going to fail because I chose a country that is a country," she said.

The professor ultimately did not apologise for her mistake but regraded Ms Arnold's assessment to B+.

The university has since removed the professor from her post and offered to reimburse Ms Arnold for the cost of her tuition.