JB Hi-Fi has apologised to a man with Down Syndrome who was blocked from entering one of its Brisbane stores on Monday.

A security guard and manager at the Mt Ommaney store refused 21-year-old James Milne entry on the grounds that he looked like another man with Down Syndrome they suspected of shoplifting.

However Milne's sister Victoria – whose original Facebook post decrying the discrimination went viral on Tuesday morning – said she was "not impressed" with the apology.

FACEBOOK/VICTORIA MILNE "I have never been so disgusted and mad in my life," said Victoria Milne.

In a statement, CEO Richard Murray said JB Hi-Fi apologised unreservedly to Milne for any distress he suffered.

"We should have done better yesterday. We are going to make sure that we learn from this and do better in the future," he said.

"I have sent a personal letter of apology to James and we are continuing to endeavour to contact the family to apologise directly."

At 3.10pm (local time), Victoria Milne updated her Facebook page saying she had spoken briefly to Murray over the phone.

"He was adamant that the manager of JB HiFi was very distressed. I was yet to see his (the CEO's) personal letter of apology to James so he emailed it through," she said.

"It included the following: 'I am proud of the staff at our Mt Ommaney store, however there is no doubt the events of yesterday are disappointing.' Not impressed."

Victoria Milne, James and their father Daryl Milne have been responding to media requests all day, and were grateful for the outpouring of public support.

"James is a happy, fun-loving guy so this hasn't dampened his spirit in the slightest, he was being pretty cheeky when the cameras were around," she said.

Cath Watson from Down Syndrome Queensland said the family had contacted them on Monday evening after the manager refused a second time to apologise to Milne's mother.

She said the public response was the silver lining to the unfortunate incident.

"As a society we've developed a fairly high tolerance now for diversity and accessibility for people with disabilities or any kind of difference," she said.

"When we see anything that is exclusionary based on a profile, we've been very quick to act and call out those people to say this is unacceptable."

Thousands of social media users across Facebook and Twitter have slammed JB Hi-Fi, even calling for a nationwide boycott.

Nicole Reaney from InsideOut Public Relations said the electronics retailer was generally popular among that demographic, and the affair had the potential to seriously damage its brand.

"We've seen it recently with the Lorna Jane situation. While people may not boycott – it has certainly changed the sentiment towards the brand," she said.

"Swift and sincere action on behalf of the company is what will determine the level of damage."

Brisbane activewear brand Lorna Jane has made headlines in recent months over its alleged sizism towards employees.

Reaney said Murray's apology should be updated with more details about how Milne's exclusion occurred and what steps will be taken next.

"This type of situation emphasises the importance of ensuring an issues management plan is in place, but more importantly is updated and practiced regularly.

"Training and education should occur at key customer-facing areas to ensure swift resolution at store level – rather than the situation exploding on social media and media outlets."

Murray told media station 612 ABC on Tuesday afternoon that it had been "a really disappointing day for us".

He said they would re-examine systems in place to prevent it happening again.

"While this is a challenging issue for James, I want to make sure that we also take the lesson across all our stores and do better across all our stores," he said.

"[So it's] probably less disciplinary and more a counselling process around revising our procedures and training to make sure it really addresses the issue."