Labour's Corrections spokesman says Serco's illegal interception of an inmate's mail highlights a lack of transparency around what goes on in prisons.
Guards at a Serco-run prison in south Auckland were forced to apologise to an inmate after opening and reading their letter to New Zealand First MP Mahesh Bindra.
Kelvin Davis says there isn't much prisoners can do.
"The bars and the gates that keep people inside can also keep scrutiny out, and there's very little that prisoners can do about it.
"They're not listened to. Their complaints get thrown in the bin. So they're between a rock and a hard place, to be honest."
Prisoner correspondence with politicians is considered legally privileged under the Corrections Act and should not be opened.
Mr Davis says prisoners' reintegration into society will continue to be disrupted if they leave prison angry with how they've been treated.
He says the law-breaking is counterproductive to what prisons aim to do.
"To be honest there's probably another couple of thousand people in prison who also need apologies for their mail being tampered with. It isn't an isolated incident. It's just really hard to prove.
"Prisoners say to me all the time that they suspect that their mail has been tampered with."
He says prison guards cross privacy boundaries because they know they can get away with it.
"If a guard wants to open something, and especially if he or she suspects it's about him or her, just take it and destroy it, and no one will know any different."
Mr Davis says there's a total lack of transparency in what goes on in our prisons.