A relative of two Indigenous children who were detained with zip-ties says the children were allegedly bound so tight they were bleeding when they were found.

Roberta Cox, a relative of the children, told The West Australian the cable ties were so tight they broke the skin and drew blood. When she saw this, along with the deep marks on their skin, she asked the police to call an ambulance.

"I said 'you guys need to ring the ambulance as well' because one of the kids was bleeding on the wrists," she said.

45-year-old Broome resident Mat Radelic has been charged with three counts of aggravated assault. He has been accused of restraining three children, aged six, seven, and eight, after he allegedly caught them swimming in the pool of a vacant house.

Two of the children appeared restrained in footage, with police saying the eight-year-old child was also tied-up but left the scene before they arrived.

Ms Cox said she asked Mr Radelic to release the children, who were allegedly forced to sit on the pavement in Broome for over an hour, as temperatures reached a high of 33 degrees.

"I heard women going off, so I eventually went across the road, and I saw a bloke in his work uniform and the two kids sitting up against the fence," she told The West Australian.

"I could see that they were very distraught and the cuffs on them were absolutely too tight."

Ms Cox, who has a background in law enforcement, alleged Mr Radelic asked her if the children were going to "pay for this unit," to which she replied: "At the moment, you need to get them in the shade and you need to give them a drink of water."

"He was refusing to let the mother walk in."

She said she called the police, asking where they were, before telling Mr Radelic to release the children.

"I told him that I'm an ex-cop and a health worker as well and that the cuffs were too tight for those kids, but he wouldn't release them until the coppers came," she said.

When the police arrived, they cut the ties off the children and questioned Mr Radelic before he was arrested.

Premier Roger Cook said the footage raised "very strong emotions" and "anyone that saw that video would have found it confronting, would have found it disturbing".

WA Police Acting Assistant Commissioner for Regional WA Rod Wilde said whilst there is room in the criminal act for a citizen's arrest, police will be alleging Mr Radelic used force that wasn't appropriate.

"Whatever force you apply to arrest someone needs to be reasonable, given the age of the person involved, the vulnerability, and all of those things that be taken into account by the court," he said, as reported by the ABC.

Noongar academic and human rights law expert Dr Hannah McGlade told National Indigenous Times on Tuesday the incident had "shown the ugly face of racism in Western Australia".

"Young Aboriginal children crying and tied up in a vigilante style manner. I hope the police use their power under the Criminal Code WA to ensure the racist element is reflected in the law," she said.

Mr Radelic will appear in court on March 25.

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Note: Previously, all coverage of this issue in National Indigenous Times has concealed the faces of the three children pictured. Recently, family members of the children have shared other photographs of the children with news media and their faces have appeared on television news and elsewhere. As such, National Indigenous Times has concluded that the family is comfortable with the children's faces being visible in reporting of the case.