Words in blood on walls of dead Kiwi woman's Sydney home

Stacey and Seth Docherty
(Stacey Docherty / Facebook)

The father of a four-year-old boy found dead in Sydney can't understand what his former partner New Zealand woman Stacey Docherty was thinking.

Ms Docherty and her son Seth were found dead in their Sydney apartment on Monday. Police are investigating if the deaths were a murder-suicide.

Messages written in blood were found scrawled across the walls of their home, Fairfax reported.

Matt Davis says his ex-partner showed no signs of suicidal thoughts.

"I miss Daisy. Besides our differences in the past, she was a very loving and caring person. I can't begin to fathom what made her make the decision she did.

"I'm going through some gnarly process but will get through this," he wrote on Facebook.

"I will post some pics of me and my son for those (many) who never had the pleasure of seeing him. I feel grateful for a brief time I got to be a father to a beautiful little boy."

Neighbours told the Daily Telegraph Ms Docherty had history of police callouts, and Family and Community Service workers were regularly involved with the family.

They say the system "failed" Seth.