Stacey Docherty told Mr Davis, who had planned to take their son to the park, to come to her place in Sydney's east later that afternoon.

She would leave the door open for him.

After he finished work and arrived at the Hillsdale unit about 1pm, his door knocks went unanswered and his world collapsed around him. 

Inside were the bodies of Ms Docherty, 38, and their four-year-old son Seth. 

Scribbled over the walls was a mess of barely decipherable messages written in blood.

Police said on Tuesday they were still keeping an open mind as to whether a third person was involved in the mother and son's death or if it was a case of murder-suicide. 

Former professional skateboarder Mr Davis conceded that, while his former partner had anger issues, she was a "beautiful mum" who loved Seth dearly. 

"She really did love Seth, she put him first before herself all the time," he said on Tuesday at his eastern suburbs home. 

Ms Docherty moved from New Zealand to Australia more than a decade ago and worked as a nursing assistant in Sydney. 

She spent five years in a relationship with Mr Davis, who said he stayed close to Ms Docherty and Seth, often visiting them at the Grace Campbell Crescent unit. 

Most recently, Mr Davis said his former partner was feeling the stress of a separate relationship break-up and having to find somewhere new to live.

Neighbours and police were also aware of Ms Docherty's volatile behaviour. Last year she smashed the windows of three of her neighbours' cars after a dispute about parking.

Mr Davis said he had spoken to Ms Docherty about her anger and encouraged her to get more support.

"She could've received more help, there could've been more help for her in regard to her anger and frustrations at the world," he said.

Despite the possibility that Ms Docherty killed their son before ending her own life, Mr Davis wasn't angry. 

"I can't begin to fathom really what made her make the decision she did," he said.

"Obviously I wished she called me or let me know she was having these feelings so I could have left work and been there to try to help sort it out and to not be in the position I am in now.

"But she didn't."

In a sign she was looking forward, Mr Davis said Ms Docherty agreed on the weekend to negotiate a custody agreement for their son. 

NSW Family and Community Services was in contact with Ms Docherty, Mr Davis said, but she always promised she would never do anything to harm Seth.

"Stacey never actually said to me anything or said to me she had suicidal thoughts," he said. 

"She vowed to me constantly that she would never do anything to hurt Seth. And I knew she wouldn't, she was very protective of him in regards to other people.

"I just want people to know that I love my son and I love Stacey in my own way and that we did our best to try to be good parents to him. 

"He deserved more than what happened to him."

Initially police were concerned about a strong smell of gas in the apartment complex on Monday afternoon. However, investigators have since ruled out that it contributed to the mother and son's death. 

Mr Davis has encouraged people with concerns about their loved ones' mental health to speak up.

"Even if you feel intrusive, be intrusive," he said. "Because the outcome is this and it is painful and it is shocking."

Lifeline 13 11 14; MensLine 1300 789 978; Kids Helpline 1800 551 800.