Sia has opened up about why she decided to adopt her two 18-year-old boys last year.

In the October cover story for Vogue Australia, the "Together" singer, 44, says she was inspired to adopt her sons after seeing one of them while watching a documentary.

"I am obsessed with reality television and documentaries, and I watched a documentary and saw my son," she told the magazine.

Sia added that she was touched by the story after realizing that the boy had no family of his own.

"I was like, ‘What? Like, he doesn’t have anybody. Oh my god. I’m going to find him and I’m going to be his mummy.’ And so that’s what I did,” she said.

Earlier this year, Sia announced during an interview with SiriusXM's The Morning Mash-Up that she adopted the two boys in 2019 to save them from aging out of the foster care system.

The singer tells Vogue Australia that being a mother is "the best thing I've ever done," but also admits that her journey into motherhood has been challenging.

“It’s one of the hardest, but I have obviously such an overflow of love that I could definitely see myself doing it again, but not for a while," Sia said, adding that she's looking into fostering infants.

"Maybe [their mother] is drug-addicted and then I could help look after them until, you know, their mum can get back on the wagon or an adoptive home is found. If I can do that then I think I’ll feel like I’m superhuman.”