Parents are being warned children are 50 times more likely to be abused when unrelated adults are living in their home.

Child advocacy group Child Matters says the so-called 'Cinderella Syndrome' has been highlighted by the charging of Matthew Ellery for the death of Ngaruawahia infant Serenity Jay Scott-Dinnington.

Ellery was the partner of the girl's mother.

Child Matters spokeswoman Amanda Meynell says international research shows children living in households with unrelated adults are nearly 50 times as likely to die of inflicted injuries.

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"When you're talking about unrelated adults being in the home, that attachment's not so great, they don't have the relationship, they may not have been there since birth with that child and so they pose a greater risk," she says.

Ms Meynell says parents should exercise common sense when leaving their children around people they don't know well.

"We're not just talking about step-parents, we're talking about anyone that might be living in the home.

"Boarders, friends, anyone that you have that's not related to that child. Be really careful about that, make sure that you believe that that's a safe person to be around children," she says.