"I open the door and there she is lifeless in my bathtub. Cold, white, unresponsive, not breathing, blue lips, dead weight," she told 9News.
"It was traumatising."
Walsh was diagnosed aged four with epilepsy and had been taking medication, but was suffering seizures more frequently after falling pregnant, her sister Jemma told 9News.
Loved ones have remembered the "beautiful" 19-year-old in tributes on social media.
"Fly high beautiful girl! I was lucky enough to have known you and been apart of your family. You will be dearly missed!" Teagan Murison said.
"So many beautiful lives getting cut short," Jarred Kenneth wrote on Facebook.
"I miss you so much I still wait by my phone waiting to see your name pop up saying you miss me," another friend posted to Facebook.
She was "strong" and "didn't care what people thought", another friend said.
A fundraiser has been organised for Sunday at the Tomago Bowling Club to help pay for funeral costs.
A GoFundMe page to raise funds for Epilepsy Action Australia has raised more than $2,200 in two days.