The gunman who shot and killed a mother and her teenage daughter in Perth's affluent western suburbs did so because he thought his ex-partner was at the property.

WARNING: This story contains details that may distress some readers.

The 63-year-old man visited a home on Berkeley Crescent in Floreat on Friday afternoon looking for his ex-wife, but instead found the 59-year-old owner of the property and her teenage daughter.

The ABC understands the gunman's former partner, who was a good friend of the family, had warned the women the man would be coming to the house, and told them to hide.

WA Police Detective Inspector David Gorton said the man shot the mother and critically injured her 18-year-old daughter.

He said police were on the street by the time the last gunshot was heard, which police believe was when the gunman turned the weapon on himself.

The mother was declared dead at the scene and the younger woman was rushed to Royal Perth Hospital, but was confirmed on Saturday morning to have died from her injuries.

Detective Inspector Gorton said the gunman was known to police, but he did not have a history of violence, and was a licensed firearm owner in the state of WA.

He said the victims' family and the gunman's ex-wife were being supported. 

"Obviously they are very distressed by these circumstances and WA Police will make the appropriate referrals to them for counselling," he said.

"Any death is tragic however when you get a multiple murder, it has that greater impact upon the family, the friends, the first responders and certainly the community at large."

Detective Inspector Gorton said investigations into the incident were ongoing, but it was not being treated as a domestic violence matter due to the victims being unrelated to the gunman.

'Close-knit' community shattered

A heavy police presence remains at the scene as investigators continue to piece together how the incident unfolded.

Speaking from her Floreat office on Saturday, Curtin MP Kate Chaney said many in the community had been left outraged by the incident.

"Two women have lost their lives because of a man with a gun, and it's a reminder that violence against women happens in every community, even right here in our backyard," she said.

"Enough is enough. So many women that I've spoken to in my community last night and this morning are feeling exhausted and furious because more women are dying at the hands of a man.

A neighbour, who did not wish to be named, said she had been hit hard by the news of the mother's death.

"It's so horrible. She was such a beautiful mother, a truly beautiful lady," she said.

"She's been living there forever. We moved in over ten years ago and she was already living there.

Another resident said he knew the family "reasonably well".

"I've met the mother a few times, she's a lovely person, so we wish the family well. People in this area are really nice, really friendly, and yeah, it's sad," he said.