The Corrections Department is set to become the largest government department in a few years in terms of staff, Finance Minister Bill English says.

Mr English told the Herald's Mood of the Boardroom conference that the number of directly employed Corrections staff would outstrip other government departments in the coming years.

"If you're wondering why the Government is spending a lot of money, that's one reason. That's our fastest growing portfolio - Corrections will be in two or three years the largest government department, bigger than the Ministry of Social Development or the Inland Revenue Department."

A spokesman for the Corrections Department said it employed 7184 full-time equivalent staff, but he did not have projections for future staff numbers.

However, he said with an expected increase in the prison muster, more staff would be needed to manage prisoners.

There are currently 9131 beds catering a muster of about 8400 prisoners, which is forecast to rise to 12,500 by 2018.

Mr English said the Ministry of Justice, Police and Corrections would have to find $400 million in the next three years to cope with bulging prisons. Those sectors, like others, have been told not to expect any new money in the coming years.

This shortfall could expand under more punitive justice measures, he warned.

"Every time you ask for harsher penalties, that shortfall gets bigger. You are part of the driver of the costs. Lock another person up that's another $90,000 (a year) plus another $250,000 capital (spending)."

The Government is already intending to have a public-private partnership in place for a new 1000-bed prison in Wiri, South Auckland.

Meanwhile the Vote Corrections report from the law and order select committee revealed that department chief executive Barry Matthews wanted to expand drug and alcohol treatment in prisons, but did not have the resources.