The private company managing Auckland's Mt Eden Prison, which failed to reach nearly half of the targets set as part of its contract, has admitted the shortcomings.

British company Serco took over the running of the prison in August last year.

A scorecard released by the Corrections Department on Thursday shows Serco fell short on a number of the 37 targets - including one escape, three wrongful releases and three wrongful detentions in the nine months to the end of April.

Only 28 per cent of sentenced prisoners had a management plan in place within the required time, well below the required 90 per cent or more.

Serco managing director Paul Mahoney told NZ Newswire: "We are taking our responsibilities at Mount Eden Corrections Facility seriously. We acknowledge that we need to improve, and we are improving."

The report card said Serco also fell short on other targets, including initial health screening, searches and incident notification, but exceeded targets for drug-testing, and rehabilitation and reintegration activities.

Mr Mahoney says the prison has one of the lowest positive drug test rates in New Zealand.

"There were zero self harm threat to life incidents in this reporting period, and the number of justified complaints remained very low. We are taking what we have learned from our successes and applying the same focus to the areas where we need to do better.

"We have a plan to get to a high level of performance and achieve 90 per cent of our measures by the end of the year."

Corrections says most of the company's results are either stable or improving since its last assessment, over the six months to January.

Corrections deputy chief executive Christine Stevenson says while Serco has fallen short in some targets, she expects gradual improvement.

"In total, half of the 37 targets were met or exceeded. Trend analysis shows that Serco's performance against 80 per cent of all performance measures is either improving or stable.

"Our contract with Serco has high targets and we expect them to be met."

Ms Stevenson praised Serco's steps to make prison visits more family-friendly, which it hopes will reduce the risk of reoffending and motivate prisoners to make positive changes, while its prisoner reception systems and facilities - including an induction for new prisoners - have "led to a significant decrease in incidents".