A mother of the two children who fled from their caregiver's home in the dead of night is being told to hand her children over to authorities immediately.

Child, Youth and Family have today revealed there are now serious concerns for the safety and welfare of missing brother and sister Te Morehu, 15, and Anahera McLean, 7, who haven't been seen since they stole away from a south Auckland property in the dead of night more than a week ago.

Their disappearance sparked a major search and rescue operation which was called off after it was discovered the pair had been picked up by a family member.

But now CYF general manager operations Kay Read has urged the McLean whanau to bring them out of hiding. She also accused the whanau of using the children as "bargaining chips" in a dispute with the department over their custody.

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"We are extremely concerned about two young children that are currently missing. Their mother has texted us to confirm they are with wider family, but will not let us or the Police know where the children are so we can be satisfied about their safety," she said.

"Our primary concern is for the safety and wellbeing of these children and we urge the family to return them safely - and to do this immediately. "

Read said any decisions around the custody of children and young people were a matter for the Family Court and made with the best interests of the children as an absolute priority.

"Children shouldn't be used as bargaining chips in an attempt to influence decisions regarding custody or placement. No decisions can be made while these children are still missing."

She said CYF had always been willing to work with the mother and her wider family and intended to continue their involvement.