About 240 people came from throughout Otago and Southland to take the chance to see first-hand the workings of the 335-bed prison.
It was the first public showing of the prison when inmates have been there.
The first taste of life in prison for visitors was a search by a drug dog.
They were then taken on tours of the prisoner processing area, the visitors area and the remand centre.
Corrections officers and management guided the 30-person tours throughout the prison.
Some remand prisoners made the most of the audience, with rude comments and the occasional bout of offensive language.
As some in one group noted, that kind of behaviour made them appreciate their own freedom.
Prison manager Jack Harrison, who addressed visitors at the start and end of the two-hour tour, said the open day was a chance for the prison to show the positive work it did.
He was pleased to be able to show the group around the complex, which began accepting prisoners just over two years ago.
The open day was also a chance for the visitors to give something back to the Milton community, with Milton Area Promotions selling hot soup, muffins and other food outside the prison.
