Private prison operator Serco failed nearly half its performance targets in its first year in charge of Mt Eden Correctional Facility, but is showing signs of improvement in reducing serious assaults and drug use.
A report on the British company's first year in charge, issued yesterday, showed that in the 12 months to June it had failed to reach 18 of 37 targets.
Of the six reported measures for which it received financial incentives, it passed four.
Corrections deputy chief Christine Stevenson said the company had been set high standards, and had shown much better results in its second year in charge.
She said in April the operator had to "lift its game" after a series of mistakes.