A woman alleged to have left her two young step-children home alone in Perth while she and their father were overseas has defended her actions, blaming a post office error for the situation.

The 28-year-old went overseas to renew her visa but couldn't take her children because of the error, she told Seven News through an interpreter.

"The post office lost my children's passports ... that's what has caused all this hassle," she said.

"If you want to leave the country to go overseas you have to have your passport, that's common sense."