A woman is being questioned by police after a man was stabbed in the chest in Sydney's south.

A man remains in a critical condition after being stabbed in the heart, allegedly by his partner.

The 39-year-old underwent emergency surgery after the incident in the granny flat of a Riverwood home about 11.45pm on Sunday.

A 34-year-old woman was arrested at the scene and has since been charged with wounding a person with intent to cause grievous bodily harm.

She was refused bail and is expected to face Burwood Local Court on Monday afternoon.

Police say the woman and man were in a de facto relationship and have one child, who was not present when the man was stabbed.

"His condition is critical so his prognosis at this stage is not exactly known," Superintendent Michael McLean told reporters on Monday.

"I'm aware that there was only one stab wound although that did as I understand it penetrate his heart, so what came from that was obviously significant blood loss."

The woman in custody is believed to have requested another person in the house to contact police after the stabbing.

It's understood no one else witnessed the stabbing.

Asked whether self-defence was an element of the incident, Supt McLean said police were investigating what prompted the woman to arm herself with the knife as one line of inquiry.