In January 2018, Sarah* found herself looking around the mess that had been left behind by a nightmare tenant she had just evicted from her house in Perth's wealthy western suburbs.

"There were clothes everywhere, all over the floor, there were condoms, needles, you name it all through the house," she remembered.

But Sarah could never have imagined that what she was about to find would uncover a crime that would send shockwaves through the state.

WARNING: This story contains graphic content that some readers may find upsetting

Three days earlier she had agreed to let a woman stay at her house, which she was leasing through a short-term rental website.

But alarm bells rang when a man sent Sarah a series of text messages.

A series of strange text messages was the first sign something was wrong. ( Unsplash: Paul Hanaoka )

"I get a message saying: 'Do you have another key to the house? The prostitute staying there has my money'," Sarah recalled.

"Then another message saying: 'I'm sorry I've smashed a window, I'm happy to replace it.'

"At that point we just decided that we needed to go over and evict them."

A house turned upside down

But the tenant refused to leave and Sarah called the police to help remove her after the woman sent text messages to her threatening to damage her property.

"We walked in, we asked her to leave, she said she had done nothing wrong, and then the police asked us to step outside and they handcuffed her and brought her out, and she was screaming and yelling," Sarah said.

"It was at this point that the constable alerted me to the fact that she was a known prostitute and that the guy she was with was most likely her pimp."

When Sarah stepped in to her house she could not believe the state it was in.

"It was like a bomb had gone off," she recalled.

When Sarah arrived at the house it was a complete mess. ( Supplied )

"Lots of lingerie, lots of nylon and they'd taken down pictures off the walls, they'd moved furniture around.

"The garage was full of suitcases, her car, clothes thrown everywhere."

The evicted tenant returned to the property later that day and filled her car with some of her belongings, including a rottweiler dog.

Piles of clothes, suitcases and boxes of belongings were littered throughout the house. ( Supplied )

Sarah was left to clean up the mess and pack up the rest of her belongings.

"I got large garbage bags and started filling them up with as many things as I could," she said.

"She had opened up some of our drawers and thrown our CDs everywhere, so we collected them. I just picked up a plastic bag and put them all in there and we spent the whole rest of the day cleaning up after her.

"Then I was going to put all the CDs back together and discovered all these SD cards in the plastic bag, all these camera cards."

A sickening discovery

That was when Sarah made the decision to put one of the memory cards into her computer and open a video file.

When she put the SD card into her computer, Sarah said the contents were "sickening". ( ABC News: Jon Sambell )

"Initially the screen was black, like the iris of the camera was closed, so you couldn't see what was happening, but you could hear voices, you could hear that they were Australian voices," she said.

"Then the picture came up and I went, 'Oh my God'. It was sickening."

The video showed a young girl being subjected to sex acts by two men and a woman.

Sarah contacted the police and officers returned to the house to seize some of the items.

"Then I got a phone call maybe two or three weeks later from the police saying that they had found the people in the video, they had arrested them and that the child was safe and that they had set up a task force to investigate," she said.

Dozens of abuse charges

The discovery of the recording device led police to charge the then eight-year-old girl's mother, stepfather and another man over her sexual abuse.

Police then set up a special task force, codenamed Mirzam, to investigate allegations that the child had been abused during organised "swinging" sex parties.

The 41-year-old mother, who cannot be named to protect the identity of the victim, admitted more than 55 charges including sexual penetration of a child under 13, indecent dealing with a child and indecently recording a child.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 11 seconds 1 m 11 s Sex abuse mother arrested

The child's 49-year-old stepfather also pleaded guilty to a string of charges.

Another man admitted to his role in the abuse and was jailed for 21 years earlier this year.

It is understood the woman who rented the house was not charged as part of the investigation.

'Brazen' acts still shock years later

Nearly two years on from the discovery, the nightmare is still sinking in for Sarah, but she said she was relieved she made the decision to inspect the memory card.

"It makes me feel like a good citizen," she said.

"Some people would not be curious enough to actually have a look at what was on them or they would not be bothered.

"I'm flabbergasted that the people in the video were prepared to put their faces on camera doing this to a child — how brazen and sickening."

*Name has been changed