According to prosecutors, the woman, who has been the man's "de facto partner" for the past several years, entered her stepson's room one day last October to talk about his driving lessons.
Talking led to tickling, which led to kissing, which led to sex, Crown Prosecutor Jackie Hartnetttold the Supreme Court.
A day later, the woman's partner set up a camera in the kitchen with which he hoped to capture "evidence of paranormal activity in the house."
Going back to view the footage, the man saw the woman and his son making out and confronted them about it.
Denying the relationship at first, the woman eventually came clean, and admitted to having had sex with the teen on at least three occasions.
The sexual encounters apparently continued for weeks after the two were discovered, but the woman now says she mistakenly believe the age of consent was 16 (it's 17 in Tasmania) and is "ashamed and embarrassed at her conduct."
Moreover, she claims she is attempting to patch thing up with her partner, with whom she shares a small child.
The woman has reportedly pleaded guilty to five counts of having sexual intercourse with a young person, and will be sentenced next week.