A protest outside Mt Eden Corrections Facility will call for an end to the running of prisons by private companies.
Public Service Association national secretary Erin Polaczuk and union organisers Willie Cochrane and Nerinda Cropp will attend the protest.
The PSA has been critical of the government's decision to hand the management of Mt Eden and other prisons to private provider Serco.
"The government has effectively handed New Zealanders a life sentence with its decision to privatise the management of our prison service," Ms Polaczuk said.
Serco has a $300 million, 10-year contract to run Mt Eden and a 25-year contract for Wiri prison in south Auckland.
"There is no getting off for good - or bad - behaviour. We, New Zealand taxpayers, are stuck with this. It's a rip-off," Ms Polaczuk says.
Ms Polaczuk will speak at Saturday's protest.
Corrections, the government department that oversees prisons, has evoked a step-in clause in the Serco contract to sort out Mt Eden after the emergence of "fight club" videos and a string of allegations about serious violence.
Corrections Association president Bevan Hanlon posted on the protest Facebook page that Serco has agreed with the association that if members want to attend the rally they will not face disciplinary action.
Labour MP Kelvin Davis is expected to talk at the protest.