National campaigned on several tougher sentencing policies including stricter bail laws, an end to parole for repeat violent offenders, life in prison for the worst murderers and longer sentences for those who commit crimes against children.
It has said it will build a new prison to cope with the extra inmates, but planning and construction is likely to take several years.
Corrections Minister Judith Collins said in the interim National would house the extra inmates in existing prisons.
She said one possible solution was double bunking -- putting two prisoners in a cell -- The Dominion Post newspaper reported today.
She said she had received an assurance from Corrections Department chief executive Barry Matthews that the department would house extra prisoners within its existing infrastructure.
Double bunking has already been used in some prisons as inmate numbers exploded over the past three years, but the largest prison officer union, the Corrections Association, says it leads to increased violence among inmates, making prisons harder to manage.
Some justice groups say the potential increase in violent and sexual assaults make double bunking impractical and inhumane.
However, Sensible Sentencing Trust spokesman Garth McVicar today said double bunking was a good interim solution.
"We've got to get back to basics, where prison is a deterrent," he told NZPA.
"It's a place where people don't want to come back to. At present it seems to be a place people volunteer to go to."
He said the difficulties of operating prisons with double bunking could be avoided by moderately increasing prison officers' powers to manage and discipline prisoners.
"Long term we need to change our prison policy that gives some authority back to the guards. Under our present prison policy the inmates are basically running the prison and that is wrong."
Mr McVicar said he favoured the idea of tent prisons for short-term prisoners.
Under such a set up, which is used in Arizona, large numbers could be housing in a small area, at a minimal cost to taxpayers.
Tent prisons would be a strong deterrent.
Serious assaults in prisons have declined in recent years despite an increase in the prison muster.
In the 2006-2007 year there were 54 serious assaults by inmates on each other.
