The former model and aspiring actor, who once boasted about dating Oscar nominee Barbara Hershey, was released on parole last year after serving nine-and-a-half years, despite a desperate bid by Corrective Services to keep him behind bars.
Under his parole conditions Monteiro, 52, is electronically monitored, required to live at an approved address and is banned from entering Randwick City, Waverley and Woollahra where his offending took place.
The Sunday Night crew filmed Monteiro soaking up the sun at Terrigal beach.
Since his release he now works as a cleaner at his brother's factory on the Central Coast — the same area where he first met Gentle in 2000, when she was 30 and he was working at a coffee shop.
"The first two or three weeks, it was just all that windswept romance, he could do no wrong, there were no chinks in the armour then," she said.
Within a month they were living together — and the violence began.
"The beatings had started. If I didn't want to have sex with him, he would just make me do it anyway," she said.
"I had no say. And he was a big, strong lad. I'm 50 kilos, I had no physical strength to combat that. [He raped me] lots and lots and lots of times. Probably about every second day. In the end, it was easier to just lay there and let him do it and get it over and done with while there's tears welling up in my face, but I knew if he saw them, it'd get worse."
Gentle said Monteiro was "pretty violent" but he was "very smart".
"He'd do things so you wouldn't get a physical bruise, he would use pillows, he would just do enough like pull your hair and drag you and kick you in the stomach so there was no marks," she said.
According to Gentle, at one point he even threatened her with a gun.
"He had me up against the wall in the kitchen and the pistol in my mouth," she said. "I can honestly tell you I know what it's like to wet yourself in fear — it's a feeling you never forget."
Unbeknown to her, Monteiro had faced rape and sexual assault charges three times before she even met him — in 1991, 1995 and 1998 — but each time he got off.
A few months after moving in with Gentle, he was again facing charges of date raping another woman at The Gap. Gentle reluctantly stood by him as he pleaded not guilty in court.
"I knew in my heart that he did it," she said. "I actually wanted him to be found guilty. I wanted them to get him. I didn't want to go home."
Monteiro walked free after the jury couldn't agree on a verdict.
When he started beating her again, Gentle called the police. He was charged with rape and assault — but he struck a deal with prosecutors to downgrade the charges and again he walked free.
"I felt really let down by the system," she said.
Another victim, Laura Dye, said she first met Monteiro in the US when she was still in high school aged 18 and he was 32.
"Nineteen years ago Simon held me hostage in Sydney for 10 days and raped me, over and over," she told the programme.
Monteiro had invited her to Sydney. When she arrived, he took her to an empty apartment.
"There was a bed upstairs. He dragged me up there and raped me," she said.
Dye said she was raped at least once a day. Monteiro would sit by the phone "like a kidnapper". Eventually he decided to let her go, driving her to the airport to catch her flight home.
The day she landed in the US, he called her. "There was no hello, it was just, 'You know you're going to be pregnant, don't you?' Click. He controlled me from afar."
Dye did fall pregnant — and due to her Christian faith, decided to go through with it.
Monteiro returned to the US, where he first demanded she marry him, before assaulting her and threatening to kill her and the unborn child.
He was charged over the threats and assaults but fled to Australia.
Gentle said "there's a whole generation of young, independent women out there now, they don't know his face. They need to be aware of this monster".
