The telecommunications giant is under fire for posting an online advertisement seeking Anglo Saxon retail assistants in Sydney.
Optus has become embroiled in a racism row after a store on Sydney's affluent lower north shore advertised for "Anglo Saxon" retail assistants.
An ad posted on Seek on Thursday stated the Neutral Bay outlet was looking for a casual retail consultant and noted it would prefer "candidates who are Anglo Saxon".
The ad, taken down on Friday morning, has been widely condemned by unions, politicians and social media users.
"Optus proudly supports diversity and employs staff representing more than 70 nationalities," Optus human resources vice president Vaughan Paul told SBS News.
"This error is completely unacceptable and a clear breach of our advertising standards and commitment to equal opportunity employment.
“We… will be investigating how this occurred with a view to taking disciplinary action against those involved.”
Mr Paul added that Optus employs more than 70 different nationalities and “proudly supports diversity”.
Optus is Australia's second largest telecommunications company.
The union representing Optus workers slammed the "blatantly racist" ad and called for an investigation into the company's hiring policies.
"How did this racist requirement end up in an ad at all?" CEPU national secretary Greg Rayner said in a statement.
"This is extremely concerning and we have to get to the bottom of how this happened."
Maurice Blackburn employment lawyer Alex Grayson said discrimination in ads doesn’t often occur, but most such incidents happen “behind the scenes”.
But she said there could be a legal case for anybody who missed out on the job.
“I think it is possible if somebody has indeed been discriminated against or not been offered this job or not gone for the job that they could mount some sort of discrimination case,” she told SBS News.
Ms Grayson added employers could be liable to pay up to $63,000 in penalties if found to have discriminated against someone on the basis of race.
State Greens MP Mehreen Faruqi posted the ad on Facebook noting "Optus Neutral Bay prefers Anglo Saxon applicants".