"There was a guy that was released from prison on Wednesday. He walked into my office on Thursday with a handful of letters from inmates describing the things that had happened to them," he said.
"These things happen day after day after day in this place behind us."
Organiser John Palethorpe is from the UK, where he says Serco has also had many failings. "Given what's happened at Mt Eden in the last few weeks, I thought, time to do something."
Prisons should not be for profit, Mr Palethorpe said, and the only dividend should a be a social one where prisoners leave prison less brutalised than when they went in, ready to contribute to society.
"New Zealanders are right to ask why their taxpayer money is being used to fund a foreign company's profit margin - and the public service they are supposed to be providing, isn't being provided properly."
Lars Erickson, who was in the crowd, said the protest was an important step for working people.
"Prisoner rights need to be supported and we need to stand together to break down divisions," he said.
Corrections has taken over the running of Mt Eden Correctional Facility while a review is carried out into Serco's management.
It's already facing penalties of at least $500,000, but the Government said it could lose its contract altogether if it's not living up to the agreed conditions.
Among the incidents which have arisen are violence, contraband, and the use of cannabis and alcohol behind bars.
This week three Mt Eden inmates were arrested for allegedly helping to run a methamphetamine ring, while a former guard is also being investigated.
The Public Service Association, which represents 3000 Corrections staff and 100 more at Serco's two private prisons, said the Government should commit to no more "prison experiments".
"As a nation we can't continue to allow private companies to come in and profit from people's misery," national secretary Erin Polaczuk said.
