Sia's estranged husband has labelled her "unfit" to parent their son.

Dan Bernad claimed the singer posed a "serious and immediate danger" to baby Somersault Wonder Bernad and alleged Sia's documented struggle with sobriety meant she was unable to offer their son a stable home.

"Sia is an unfit and unreliable parent struggling with substance abuse and addiction, rendering her incapable of providing safe or stable care for Summi," Bernad, an oncologist, stated in legal documents filed this week in a request to be granted sole custody of one-year-old Somersault.

"I am the only safe and reliable parent for our son. I am a doctor, young, healthy and have no criminal history or drug or alcohol addiction issues."

He also claimed Sia had never taken on a "primary caregiving role" for Somersault, adding she "delegated nearly all child-rearing responsibilities to a revolving team of nannies while she maintained control from a distance".

In response, Sia, 49, accused her estranged husband of "weaponizing" her history of substance abuse.

"I have been fully sober for over six months and working my program which includes weekly testing and a sober companion," the filing read. "My recovery has been a cornerstone of my life and one of the primary reasons I chose to separate from Dan, whose ongoing engagement in a nightlife and recreational drug-use lifestyle is incompatible with a healthy environment for our child."

Sia filed for divorce in March after two years of marriage.