Staff then explained it was a "food and beverage issue" and breastfeeding was not allowed inside the gallery because there was the potential for milk spills.

"It was absolutely humiliating," mother Christie Rea said of the incident.

"Women who breastfeed shouldn't be shamed into sitting away in a back room."

Mrs Rea, from Newcastle, was visiting the gallery in late December with her husband and four children when she noticed her baby daughter was hungry.

"I think we were in the Versailles exhibition. I knew that my baby was going to need a feed and I didn't want to be stuck in a room downstairs."

But the instant she sat down and began feeding a security guard approached, Mrs Rea said.

"I feed in public quite regularly. I'm a mother-of-four so I'm discreet as well. I know how to be modest about it.

"Basically [the guard] said there was a parents' room downstairs and I should use that.

"He was not rude or anything, but he was quite stern. It was very clear that it was an issue of decency and modesty."

The guard agreed to let her continue feeding when she covered up by draping a piece of clothing over her shoulders.

"I put a wrap over my shoulders and he left me alone," she said. "I was shocked. I was basically shamed into covering up.

"My eldest child pointed out that there was a naked sculpture of a breastfeeding woman inside the room."

A photo of Mrs Rea breastfeeding inside the gallery clearly shows a painting of a naked woman in the background.

Less than two years ago the National Gallery held its first ever naked tours for a James Turrell exhibition as a way to "completely remove all material barriers between artist and audience".

Following the incident, Mrs Rea asked staff on the front desk to explain the gallery's breastfeeding policy.

"They initially said you can breastfeed in the foyer or in the parents' room, but we'd prefer it in the parents' room.

"They made it seem like breastfeeding fell under the food and beverage policy. It was a food and beverage issue because of the potential for milk spills."

Mrs Rea said she was speaking out about her experience in the hope the National Gallery would clarify where it stood on breastfeeding and other mothers would be spared the same embarrassment.

National Gallery of Australia deputy director Kirsten Paisley apologised for the incident and said mothers can feed in the gallery anywhere, any time.

The National Gallery was in contact with Mrs Rea on Friday afternoon to apologise in person, and have addressed staff to ensure the gallery will support breastfeeding in the gallery.

"We're sorry this has happened to Christie," she said.

"The NGA supports breastfeeding and all mothers are welcome to feed their babies at any time and anywhere in the gallery.

"We are working with all our security and front-of-house staff to ensure adherence to the NGA's policy in respect of this matter.

"I would like to express my sincere regret to Christie in person."