"I've heard it before," she told ABC TV.
"Comments like the ones on the Facebook groups aren't all that unusual.
"And because they are not all that unusual as time goes on they seem less and less obscene. So it feeds back into itself."
Ms Brookes quit the academy after just one year because she was disgusted with the way a friend who had been raped was treated by her peers.
In particular she was upset with one fellow cadet who argued the victim "should just get over it already".
Ms Brookes acknowledged on Wednesday that everyone had a right to a private life and it was difficult for Defence to intervene when it came to social media.
"But I think in terms of the attitudes that they have and their own sense of dignity, one should hope that Defence would have the capacity to engage in training and make it clear there are standards of behaviour for members in Defence," she said, adding external pressure was probably needed to drive real cultural change within Australia's armed forces.
Comment was being sought from Defence regarding the allegations.
Army chief Lieutenant General David Morrison told ABC TV in a statement that the issues raised were "serious".
He said army headquarters was working to determine if any serving members were linked to the offensive comments posted on the Facebook page.
"Where they have been I intend to take action to deal with them to the extent that our policies and the laws allows," Lt Gen Morrison said.
Defence Minister Stephen Smith launched six reviews into defence culture last year following a number of sex scandals.
A report into the treatment of women at ADFA released in November found that Australia's premier military officer education institute remained rife with low-level sexual harassment of female cadets but was much improved on the 1990s when high-level sexual misconduct was rampant.
A separate report into the so-called Skype scandal at ADFA in March 2011 remains under consideration.
