The birth of her first child should have been the happiest of times for Sydney mother Phuong Cao but friends say it marked the beginning of when her life began to unravel.

It culminated 17 months later, when on Monday afternoon the body of Ms Cao and her baby, Sienna, were found inside a home at Campsie in a suspected murder-suicide.

"She was always acting normal around her friends but you could see she was struggling a lot," immigration lawyer and family friend Tim Pham said.

"Phuong was always taking care of baby Sienna. She did everything a loving mother would do but her mind was not clear, she went down a track of no return. She was trapped. She had nowhere to go."

Born in Vietnam, Ms Cao moved to Australia with her sister more than seven years ago and had become an Australian citizen.

She was working as a nail technician when she met a man, Quyet Nguyen, and the pair started dating. Only months into their relationship she discovered she was pregnant with Sienna.

Her family says Ms Cao's challenges began when Sienna was born visually impaired and required on going medical attention.

Ms Cao's family said Mr Nguyen would often go away for days at a time, leaving Ms Cao to care for Sienna alone.

When Sienna was six months old, Ms Cao discovered that Mr Nguyen had an estranged wife and a son living in Vietnam. His estranged wife would from time to time call Ms Cao and the pair would have heated arguments over the phone.

At the same time she learned that Mr Nguyen also did not have permanent residency and she would have to help him apply for a visa if he was to stay in Australia.

"It was a shock for her to find out all of this," Mr Pham said. "It added to the stress and pressure she was already under trying to care for her newborn baby."

The relationship between Ms Cao and Mr Nguyen began to fall apart, Mr Pham said. She and Sienna left the Campsie home they shared with Mr Nguyen and moved in with her sister and her young family in Hurstville.

Detectives investigating this week's suspected murder-suicide are now aware Ms Cao and Mr Nguyen had separated and Ms Cao's mother had arrived from Vietnam to help her daughter.

On Monday, Ms Cao is believed to have driven to Mr Nguyen's home and killed Sienna before killing herself. Mr Nguyen's sister discovered their bodies.

"She thought that if she wasn't around, there wouldn't be anyone to care for Sienna," Mr Pham said.

"She didn't want to put any burden on anyone in her family to look after her baby."

Government agencies also know of at least one previous attempt by Ms Cao to self-harm.

A neighbour in Fitzroy Street, where the tragedy unfolded on Monday, said an ambulance arrived at Ms Cao's home three months ago to treat her.

"She took something and I saw her going something like crazy," the neighbour, who didn't want to be named, said.

"After one month she is gone for holidays or going to her sisters. She doesn't live here."

Mr Nguyen's friend, Dang, said earlier this week that Sienna's father was very upset and struggling to understand the tragedy.

"I don't think he has understood really what has happened yet," he said.

"I think he is still in denial."

Lifeline 13 11 14