A former Rotorua woman who lives near the scene of a grisly triple homicide at a Perth home says the killings have left the close-knit community shocked and in disbelief.

Sarah Bell moved to Perth from Rotorua in May 2015 and lives in Ellenbrook, just 150m from the Brixton Cres house where a mother and her 8-year-old son were found dead on Saturday. The mother's 15-year-old daughter died of critical injuries as she was rushed to hospital.

A 19-year-old man has been charged with murdering three of his family members.

He appeared in the Midland's Magistrates Court today and has been named as Teancum Vernon Petersen-Crofts. He was remanded into a mental health facility until his next court appearance.

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Bell told the Rotorua Daily Post she and her Kiwi partner Brad lived in a new part of Ellenbrook that had new houses, manicured lawns, lots of parks and was very family friendly and quiet.

"So I was shocked to see police cars as I was coming home from the gym early Sunday morning.

"We walked the dog through our usual route on Sunday morning, which is Brixton Cres, and were stunned to see the area cordoned off and such a heavy police presence."

She said they spoke to a neighbour who expressed disbelief at what had happened.

"We could see detectives door knocking and news crews everywhere."

She said there were lots of New Zealanders in the area and they tended to stick together.

"For example, our local bowling club is managed by a guy and his family from Rotorua, so you get to understand how small of a community it really is."

She said she was personally "shocked and extremely saddened" a mum and two children had been killed, particularly because they were Kiwis.

"I have no doubt the Kiwi community in Ellenbrook and Perth will rally around this family and friends and support them in whatever way we can, it's that sort of community.

"Brad works away a lot and as soon as the news broke I had an Ellenbrook local message me to see if I was okay and that I wasn't home alone and if I needed anything to contact them."