Sometimes, being inside a New Zealand prison is like being at the heart of a Soviet-era institution. Like walking the austere corridors of an Eastern Bloc hospital with over-zealous security.

But often you get glimpses and reminders that these buildings are home to a growing proportion of our population – on the week last month when we visited two notorious jails, the muster hits 10,000 for the first time.

As part of a Stuff Circuit investigation into the state of our prisons, we're being shown a double-bunk cell inside a low-security unit at Spring Hill prison in the Waikato.

TOBY LONGBOTTOM The Stuff Circuit investigation in to New Zealand's prisons starts on Sunday night.

There's a guitar on one of the beds, toiletries on shelves, plastic plates and utensils, and some books, one of which is propping up the weirdest television you ever did see, its components encased in a see-through shell to prevent it being used as a hiding place for contraband.

Off camera, the inmate whose room it is approaches us. He's determined to tell us some home-truths, ignoring the fact senior managers guiding us around are desperately willing him to shut up.

When the water runs (there's a toilet, a sink and a shower inside) a horrible stink comes up through the pipes, he tells us. And in summer, the cells become stiflingly hot.

PHIL JOHNSON/FAIRFAX NZ A prisoner at a low-security unit in Spring Hill Prison on the phone.

An ex-inmate has told us the same, and blames the double-bunking accommodation for violence and for robbing prisoners of dignity.

"You're in the cell with another grown man sometimes for 23 hours of the day, eating, showering and shitting together."

The man whose cell we're visiting today is middle-aged, with long curly dark hair pulled back off his face, wearing grey prison-issue singlet and shorts. He tells us he's inside for a long lag over cannabis offences - and notes how ridiculous it is he's serving prison time at the expense of the state (about $100,000 a year) for dope. Many would agree with him.

PHIL JOHNSON / FAIRFAX NZ Increasing numbers of prisons at Spring Hill and New Zealand's other prisons are required to double-bunk, as the prison muster passes 10,000.

But while the number of sentences for drug crimes has been dropping overall, those who do get sent to prison go there for longer. Increased sentences, fewer releases on parole, more people denied bail – it's a simple recipe driving our imprisonment rates, fuelling a building boom in the prison estate.

That's just one of the things we've been examining.

Today, we're with Corrections chief executive Ray Smith who has promised to show us the warts-and-all version of his domain.

He takes us to the scene of one of the lowest points in recent New Zealand prison history, the notorious Bravo unit at Spring Hill where prisoners rioted in 2013, causing $10 million worth of damage.

Inmates were boozed up on homebrew alcohol – made from fermented fruit, jam sachets and alcohol-based hand sanitiser. Since that day, we discover, the offending hand sanitisers are gone, and fruit is now being rationed for all prisoners. In jail, though, there's always a hack-around: we're told new generation yeast is being smuggled in, hastening the brewing process.

You've got to admire the ingenuity. "I wouldn't want to drink it," says Smith. "But I guess if you can ferment something you can make home brew."

Outside one of four gates it takes to get into (and out of) Bravo unit, Smith reveals to us that the sprinklers were overwhelmed the day of the riot as fires burned out of control for about 11 hours – it's a miracle no one died, including staff who found themselves embroiled in a frightening ordeal they never signed up for.

Inside the control room, we meet some of the staff. The day of the riot, enraged inmates advanced on this room, trying to break-in until the prison officers were able to flee. Even though changes have been made to the design to prevent that happening again, you imagine it must have been terrifying when it did.

This year, Corrections is on a recruitment drive, having to hire an extra 600 staff to keep up with growth. It can be a tough job, but many of the staff we encounter enjoy their work. At Spring Hill, in particular on the low security units, there is an apparently easy rapport between guards and inmates.

It's more tense at Bravo unit. Prisoners are shouting and banging walls for attention.

In the corner of the yard, one man is shadow-boxing an unseen opponent.

It's a reminder that Smith and his department cannot escape the shadow of the fight clubs scandals that rocked Corrections last year.

ENGLISH v COLLINS

The debacle of organised fighting, which focused on Mt Eden but was found to be taking place in public prisons too, resulted in Serco losing a contract and the Minister of Corrections, Sam Lotu-Iiga, losing the portfolio.

Ironically, the removal of Lotu-Iiga has set up an intriguing political stoush between the two main contenders for Prime Minister this past week, revealing one of the key policy differences between them.

The replacement for Lotu-Iiga was Judith Collins, renowned for her tough on crime approach. Her rival for John Key's soon-to-be vacant Beehive office was Bill English, who has a far more liberal view.

In a 2011 speech, English described prisons as "a fiscal and moral failure".

"Building more of them on a large scale is something I don't think any New Zealander wants to see, they want a safer community, protected by the worst elements of criminal behaviour but they don't want a prison colony," he said.

In an interview with Stuff Circuit (before Key's resignation triggered the leadership race), we put that quote to Collins.

She stopped and said: "Well, what do you want me to say?"

"It is a fiscal failure because we have to pay as taxpayers for what other people have done. As to the moral failure quite clearly the prisoners have failed, morally, because otherwise they wouldn't be there."

Challenged that that's not what English meant, Collins is not keen to engage. "Well, that's the way I interpret it."

During the interview, Collins showed no sign of softening her stance. "If people don't want to offend, they don't want to go to jail, that is the best way to keep our jails empty."

English, meanwhile, continues to show no desire to lock up more prisoners.

It was a stand-off between the two people who could have, would have been Prime Minister, over one of the most troubling social conundrums facing the Government.

A GROWING PROBLEM

Make no mistake: the growing prison muster is tied directly to Government decisions.

Analysis by Stuff Circuit for this series illustrates the link between a change in the bail laws in 2013, making it harder for people to stay out of prison prior to trial, and the increasing muster.

This year, remand inmates make up 27 per cent of the overall prison muster. In 2013, it was just 18 per cent.

There are other impacts that tie back to policy too.

In a six-part video documentary series starting tonight, the Stuff Circuit team will examine all the causes for the growing prison muster and the impacts it is having. We will reveal what really went on in some of the biggest controversies within the prison system, expose some worrying new ones, and ask: is there a better way?

* Private Business, Public Failure: Inside Our Prisons, a six-part Stuff Circuit documentary series, starts tonight at 6pm on stuff.co.nz. Stuff Circuit is a team producing video­-led, quality long­form journalism for Stuff. The team includes Paula Penfold, Toby Longbottom, Phil Johnson and Eugene Bingham, who have a background in high-­profile investigative stories.