Private prison operator Serco says it is committed to improving after failing nearly half of its targets in its first nine months in charge of Mt Eden prison.

Serco managing director Paul Mahoney says the company is "working very hard to improve" after a critical report found it had allowed a prisoner to escape in its first months in charge, had wrongfully released and detained inmates, and had not kept serious assaults under control.

The Corrections Department report found on Serco had failed to meet 15 out of 37 targets set by the department in its management of the Mt Eden Corrections Facility, New Zealand's first private prison.

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