A young mother and her 17-month-old baby girl have been found dead by a family member in their Sydney home.

The pair were discovered at their home in Fitzroy Street, Campsie, on Monday afternoon.

Campsie Local Area Command Superintendent Michael McLean has told reporters at the scene that the dead woman, aged 29, and her daughter were believed to have suffered asphyxiation.

"Early investigations at this point suggest that both have died as a result of asphyxiation," Supt McLean said.

"There are obviously a number of people who need to be interviewed and family who need to be notified so I am unable to make any comment as to the cause of death at this stage."

Officers and paramedics attempted to resuscitate the baby but she was declared dead on arrival at St George Hospital.

The mother was pronounced dead at the scene.

A neighbour has described seeing an older male relative, whom he believed to be the toddler's father, collapse on the ground in tears before detectives led him away.

"He tried to run down there and they just pulled him back, they calmed him down and took him around the corner," the man, who gave his name as Andrew, told AAP.

A number of people believed to be friends and relatives of the deceased have gathered at the scene, which remained taped off on Monday evening.

They declined to answer questions when approached by AAP.

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