The number of prisoners behind bars in New Zealand is expected to be the highest ever within weeks, Corrections Minister Judith Collins says.

At Monday unlock this week, there were 8434 people in prisons or police stations - just 23 below the previous peak of 8457 prisoners on 17 September 2007.

"Within the next couple of weeks it likely that we will have more people behind bars than at any other time in New Zealand's history," Ms Collins says.

"We are now in a race against time to ensure there are enough beds to accommodate forecast numbers of prisoners. 

"Due to a lack of planning by the last government, our prison system will run out of baseline beds around February next year. To ensure the safety of the public, we must have enough prison beds to cope with the demand."

The strong growth in the prison population began in 2003 when the prison population stood at less than 6000. It is forecast to rise to around 10,700 by 2016.

The Government has taken swift action by asking the Department of Corrections to speed up the process of adding additional modular or container cells to prisons.

"However, that alone will not be enough and double bunking will also be necessary to manage this serious capacity crisis. Double bunking is the fastest way to add extra capacity," Ms Collins says.

The Government will address the capacity crisis in three stages. Short-term needs will be met by double bunking and prefabricated or modular cells. These include converted shipping containers which could be bought and installed for as little as $53,000 to $63,000 per bed.

Medium-term capacity will be met by extending existing prisons. Longer-term capacity needs will require a new prison - most likely in Wiri, South Auckland.