"Please refrain from allowing your child to create excessive noise immediately and into the future. You must be mindful of other residents and keep noise levels to a minimum at all times," the letter read.
"Continued breaches of the by-laws may result in the matter being taken further and a fine of $550 being imposed by the NSW Civil & Administrative Tribunal."
Ms Mayer, who is expecting her second child next month, says she feels discriminated against by the letter.
"WTF?! Discrimination to the utmost! I am revolted," Ms Mayer wrote on Facebook to friends who voiced their support.
She told the Daily Telegraph she and her husband are always mindful of her neighbours, close their windows and avoid common areas, explaining: "I don't know what else we can do."
Ms Mayer says despite initially feeling distressed by the "threatening tone" of the letter, her family is now being inundated with support.
"Everyone's disgusted, especially people with children who just know that is normal behaviour for a toddler. In addition to the fact that city rental prices are so ridiculous anyway, it just adds to the feeling of being pushed out."
She and her husband no longer feel comfortable living in the building and are considering leaving the city completely.
They also have serious concerns about how the neighbours will react when their newborn arrives in April, leaving them stressed and upset.
"I don't know where young families are supposed to go," Ms Mayer told the Daily Telegraph.
The strata company Bright & Duggan told the Daily Telegraph it was a private matter so they "can't be discussing other people's correspondence", adding it was "a matter between ourselves and that particular [resident]. There are by-laws that relate to the building and it's as simple as that."
- Daily Mail