There are warnings of corruption and increased violence if New Zealand's newest prison is run by a private company.
Critics say the new men's jail in South Auckland will be all about profit and there is also no incentive to rehabilitate.
"We're disturbed that in New Zealand we seem to be trying something that's been tried and tested and failed," says Richard Wagstaff from the PSA.
It's a concept that has been tested in Australia, the United States and Britain and Wagstaff says those same private prisons have problems with inmate escapes, high reoffending rates and are more violent.
"They score badly on just about every count .. and certainly the concern is they have an interest in people staying in prison because they make money out of that and so we don't think the incentives are right," Wagstaff says.
The 1,000 bed mens prison, to be built in Wiri, should be operating by 2014 as New Zealand's first public-private partnership.
The government says it expects to save between 10 and 20% over the prison's lifespan.
But when the Auckland Central Remand Prison was run privately by Australians for five years it cost $43,000 per inmate. That is $7,000 more than when it was run publicly.
GEO, the Australian company that ran the Auckland Central Remand Prison, has confirmed it will be bidding for the new jail job.
In a statement it says the company is keen to re-enter the New Zealand market and would welcome the opportunity to be part of the government's tender process.
But also in Australia comes criticism of other private prisons facing problems with staff and security.
"To make profit they have to take shortcuts somewhere and the first place they take shortcuts is the staff on the ground," Parklea Prison Officer Matt Binley told Australia's ABC.
The PSA says the government needs a short sharp rethink before the public are forced to serve time for their mistake.