Badler wrote on the Instagram photo sharing website: "He was 27 years old and the world was at his feet, but unfortunately he was suffering from mental illness and addiction ... I will miss you, Harry every day of my life."
Harry Haynes is best known for starring the American Horror Story Hotel and The OA.
It is not known whether his death was caused by suicide or an overdose of the drug.
Harry had posted a festive New Year's Eve photo with his girlfriend on Instagram, and she is considered the last post on social media.
He wrote, commenting on the photo: "Until 2020! A new era has already begun ... So let us continue to move forward only, and we remain present to enjoy every moment throughout this beautiful new start, a happy new year, a lot of love and gratitude for everyone in my life."
Harry grew up in Melbourne, studying there until he decided to move to London to pursue fashion and acting shows, and then moved to New York, before settling in Los Angeles.
In addition to starring the American horror film, Harry signed a contract with the Chadwick Fashion Agency, and was also a musician performing his songs under the name ANTIBOY.
He had previously admitted that he had a "severe sleep disorder" as a child, and he had a tendency to "self-destruct."