Detectives have confirmed a note was found on board the vessel.

WARNING: Distressing content.

Police have revealed disturbing new details about the drowning deaths of a six-year-old girl and her father in Sydney’s inner west, saying the 47-year-old man had planned and rehearsed the murder-suicide.

NSW Police give update after body found in Sydney's south-west - ABC News

An empty boat was found drifting slowly across the Parramatta River on Saturday, after the father deliberately threw his young daughter overboard before jumping in himself. Neither could swim.

WATCH THE VIDEO ABOVE: Father drowns daughter in planned Parramatta River murder-suicide

Detectives discovered a note on board the vessel, confirming their suspicions that the tragedy was premeditated.

Investigators have since learned it was the third time in a week the man had hired a runabout from the Cabarita Point Marina.

The father and daughter arrived at the marina in an Uber at 10am on Saturday. Just before midday, boaties discovered the man’s body floating in the water.

Divers recovered the little girl’s body at dusk.

“This is an absolute tragedy for the family and the community on every level,” Burwood Police Area Command Superintendent Christine McDonald said.

While concerns about the father’s state of mind have since emerged, police say they received no formal reports of family or domestic violence.

“It’s up to everyone to speak up if something seems wrong,” federal domestic, family and sexual violence commissioner Micaela Cronin said. “Watching for behaviours that you are concerned about and asking: ‘are you OK’?”

“Unfortunately, it’s not uncommon for there to be people who are struggling in silence.”