A $218 million prison development towering over the Southern Motorway and casting a shadow across Mt Eden has horrified residents, principals and community leaders.

They are appalled by the project at Mt Eden Prison, and one city councillor believes the new block has grown higher than initial plans indicated it would.

The building, due for completion by April, will provide 554 additional beds - and support facilities that will be shared by the Auckland Central Remand Prison.

Many of the cells will be above the level of the adjacent motorway.

That is of concern to Mayor John Banks, who last night blasted the new-look jail as a "an architectural monstrosity".

"For people coming into the city, it says: 'Welcome to the aspirational capital of New Zealand, where you jail people ... and it's great'," he said.

"I can only hope that we are not going to see people with their noses pressed against the bars.

"It just should not have happened and I railed against it before it rose from the ground."

However, the Corrections Department said last night that windows would be frosted and inmates would not be able to see through them. There would also be "acoustic buffering" to dampen noise created by rowdy prisoners.

Neighbouring Auckland Grammar is also worried by the development. In a letter to Corrections chief executive Barry Matthews, board of trustees chairman Jeff Blackburn raised three issues over the "scale of the redevelopment" on the site.

These were the security risks posed by a shared boundary fence, the prospect of shade ruining sports fields and the potential for prisoners to look onto the school's grounds and for sound to carry from the new tower.

"It will add a degree of shading which may make the provision of winter sports fields impossible," Mr Blackburn wrote.

A Corrections spokeswoman said the department intended to meet Auckland Grammar leaders "and is committed to working through their concerns in an open and constructive way".

Councillor Glenda Fryer, who represents Mt Eden and Mt Albert on the city council, shared the mayor's dislike of the project, saying that what had once seemed like a good idea - to build upwards and therefore maximise prison space - was now "off-putting".

"It is a well-attended school and the fields are really important. And if it's being overshadowed, it means that grass isn't growing properly. They [the fields] become more muddy, and that isn't good for sport."

Mrs Fryer said the original draft plans the council was shown for the project had the tower at the same height as the motorway.

"I was driving there [yesterday]," she said, "and I said to the passenger in the car that we were told it was only going to be the height of the motorway - and here it is five storeys above it."

The principal of nearby St Peter's College, Kieran Fouhy, said the prison redevelopment was "a crazy waste of money".

"Philosophically, it seems crazy that you're paying $100,000 a year to fund one prisoner when you could fund 20 rugby teams across the road, you know, or two teachers."

Mt Eden resident John Kaminski, 20, a Grammar old boy, said prisons should not be so close to town.

"The main visual feature around here is going to become the prison ... We don't want to portray having a jail in the middle of town ... If they do want to build a jail, why can't they build it out by the airport?"

Newmarket Business Association chief Cameron Brewer was a lone voice of support for the project. He said the $250 million-plus being spent on upgrading the site "is arguably going to improve the area aesthetically".

"What [the] block rising from the ashes does is clearly remind people travelling through Spaghetti Junction in particular that there is a prison in the neighbourhood."