There are concerns the treatment of youth offenders in Serco-run prisons, could lead to more criminal offending later.

The Ombudsman's warned urgent action is needed after it emerged teenagers at Mt Eden prison are being kept in their cells for up to 23 hours a day.

Labour's corrections spokesperson Kelvin Davis says that risks doing permanent damage.

"They're still developing emotionally and for them to be locked up for that amount of time in a small cell, probably with somebody else - it's just not good for them."

He says any problems that might develop from that could make it harder to turn troubled youth into productive members of society.

"When they get out, what we want for them is to be able to reintegrate into their society and community and not be damaged."

Green MP David Clendon wants to know why the hours of lock down for young offenders increased "despite corrections being alerted to the problem four times in 14 months between April 2014 and June 2015."

Corrections Minister Sam Lotu-Iiga said the number of hours had increased since then and called it a positive.