A 17-year-old girl was among more than 20 Extinction Rebellion activists strip-searched in the Brisbane City police watch house recently during a week of climate protests.

On 9 October, the third day of Extinction Rebellion civil disobedience, a protester had brought a phone undetected into her cell at the watch house and livestreamed video for about 15 minutes to a private Facebook page.

Police warned the woman she would be strip-searched if she was arrested again, but responded the following day by repeatedly searching, then strip-searching, every activist arrested.

One of those activists, a 17-year-old girl, told Guardian Australia she had questioned police about whether they had the right to search her.

“I said I’m 17, can you strip-search me?” the girl, who cannot be named for legal reasons, said. “They said actually it doesn’t matter.

“It was done in a private room at the watch house ... the thing is when you’re in the watch house it’s already dehumanising, you lose all your right to any dignity. It’s not great, it really isn’t.”

Strip-searches of young music festivalgoers and environmental activists could wake up middle-class Australians to a practice that routinely affects the poor, Indigenous and disadvantaged, a leading advocate for the rights of prisoners has said.

Debbie Kilroy, the founder of the organisation Sisters Inside, likened the practice of strip-searching to “sexual assault by the state”.

“If I did that to you, or you did that to me in the street, we’d be facing serious criminal charges,” Kilroy said. “So why do we allow it to happen in prisons and watch houses?”

Her comments come after revelations in the NSW Law Enforcement Conduct Commission that police strip-searches, including searches on a 16-year-old girl at the Splendour in the Grass music festival, may have been illegal.

Kilroy has been campaigning for a decade to end the practice in prisons and police custody. She says where some might be shocked to hear about minors being subjected to strip-searches, the reality is that they are used by authorities on a daily basis.

“It’s predominantly Aboriginal and Torres Strait Islander people who are criminalised, it’s predominantly very poor people, it’s homeless people, it’s people with mental illness,” Kilroy said.

“They already know what the experience is like.

“The more we see young people and people on the streets protesting … there’s going to be more and more people exposed to police brutality. We’re going to see middle- and upper-class people on the streets who are concerned about the climate crisis. We are seeing already laws enacted to criminalise them.

“Now the middle class is being targeted by the expansion of laws to control them and gag them. Their eyes are going to be opened by this. The experience will connect the divide.”

Kilroy said the way certain groups are criminalised or viewed with automatic suspicion feeds into decisions by authorities to conduct searches, which they can do lawfully where they have reasonable suspicion.

“In prison, if you come to prison for whatever reason, there’s an automatic suspicion. Across the country in prisons it’s a common practice. Based on their own statistics, it actually doesn’t uncover the contraband and the weapons. That’s actually a myth. It’s about social control and the further sexual assault of women by the state.

“We know the high numbers of women and girls who are sexually abused or raped. These women get strip-searched in a way that is retraumatising them, clothes ripped off them or cut off them.

“If you’re menstruating you have to remove your tampon or your pad and hand that over.

“These women [might] try to protect themselves, but police put it in a frame where they’re attacking police, but they’re actually trying to protect themselves and stop being violated.

“This is nothing but sexual assault by the state by prison officers [and police] who do this day in day out.”

Kilroy said the practice happened at prisons which had airport-style scanners that could easily be used to replace invasive searches.

Several Extinction Rebellion activists spoke with Guardian Australia about being strip-searched after being arrested on the Thursday of Rebellion Week. Each said they had already been throughly pat-down searched by police before arriving at the watch house.

“It was a full pat down, right up under my bra, in all my pockets, there was no way they could have missed anything,” said Vicki, 49, a mother and childcare worker.

“It was pretty shit. It was pretty shocking because I had no understanding of why I was being strip-searched. They didn’t say to me ‘do you have anything on your body that is dangerous?’”

Vicki and other activists said the searches seemed like a “knee-jerk reaction” to the embarrassment caused to police by the livestreaming, and that police had been unnecessarily aggressive to people who posed no threat.

“We don’t want to hurt people, our biggest thing is we’re non-violent protesters. Police are responding to peaceful protesters with violence.”

Police said in a statement the safety and security of people in custody, watch house staff and support agencies was paramount.

“When a person is admitted to a watch house they are searched according to legislation, policy, procedures and information known to police,” the statement said.

“People are searched by an officer of the same sex in a private section to maintain the dignity of the person being searched.

“Children are searched according to the same standards however a higher threshold is applied in acknowledgement of a child’s vulnerability.”