Bayfair is investigating the incident but its centre manager believed the security guard was not acting out of malice.
But in Duggan's view, she felt the security guard had made a scene by shouting across the mall, "Excuse me. Buy a shirt or leave".
"I was mortified. I was so upset I just left," she said.
"I said, 'That's why I'm in here because I don't have my singlet. We're not from here so I can't just go home and get one'.
"He said, 'You have to have a shirt, Kmart is right there'. I said, 'If you had been following me on CCTV footage you'll see I'm going to the more expensive shops because I'd like to buy an outfit, not just a shirt'.
Duggan, who was born and raised in Tauranga, went to her car where she was close to tears while her husband approached the security guard.
According to an entry sign there is to be "no smoking or vaping, riding, bikes, alcohol, loitering and gang regalia".
After taking a photo of the sign, Duggan's husband approached the security asking where the "no swimwear" rule was on the sign.
She says the security guard admitted the no swimwear policy wasn't on the sign, but told her husband the rule came from the owner.
"I work, I pay my taxes, I give back to the community. I was born and raised in Tauranga. It really upsets me. I'm embarrassed, " she said of the incident.
Bayfair centre manager Steve Ellingford said the incident would be investigated but he believed the guard was not acting out of malice.
Ellingford said the dress code was not on the front door of the shopping centre as it would not be practical to post every rule on the door.
"It's about appropriate dress, which covers everyone. We are a dining, entertainment and retail precinct so there is a certain level of dress we would expect," he said.
"It's not meant to be offensive to anyone, it's just having a line somewhere where we say, we would like people to dress to a certain standard," he said.
Men walking around without a shirt on would be asked to put one on, too, he said.
But he said given the circumstances, and the fact the woman was there to buy a top, he would not have asked her to leave as it was a "fair and reasonable answer" to why she was in a bikini top.
"Whether the guard made a mistake, I don't know ... it's just about trying to understand that."
He said the guard would "absolutely not" lose his job unless he was abusive or crossed the line, in which case a disciplinary process would take place.
"We all make mistakes."
Ellingford said he had not been in touch with the guard due to the long weekend.
He said the woman had not, to his knowledge, approached the centre but he would be happy to speak to her about it.
