Demolition workers working on a Hamilton building were lucky not to have been seriously injured after part of a roof and wall collapsed on site in the central city.

Nearby workers say they heard a massive "rumbling" and a loud bang when a section of wall about 20m wide and a 20m by 4m piece of roof suddenly collapsed after a strong gust of wind hit just after noon today.

Waikato Demolition have been working on dismantling the 1920s Railway Building - next to Centre Place Mall - on Victoria St through this month.

The scene of the incident. Photo / Belinda Feek
The scene of the incident. Photo / Belinda Feek

Waikato fire area commander Roy Breeze says the company had just removed a bit of the wall and some of the roof when a strong gust of wind has come through.

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"Just the leverage of the roof lifting up has made the rest of the building, the corner and front arc, unstable and some of it broke away and collapsed. Fortunately no one was working right in that immediate area so they escaped injuries."

Mr Breeze says the building remains "quite unstable" with the "danger area" still vulnerable to wind gusts and potentially drop down onto the footpath below.

There was now a designated safe area for staff to work in and he had been assured by demolition staff that no one would enter the danger area until it was made safe.

"I don't know how close [staff] were but they were close enough to be in danger. It was certainly an unexpected collapse so I think we're lucky that no one got seriously injured in this."

John Wheaton, area manager of Timezone which sits across the road from the building, says he heard a massive "rumble, rumble, rumble" for about four seconds and then ran outside to see the wall and roof had collapsed.

"I was just sitting in the office ... and come outside and the whole side of the building collapsed. They were renovating it and may be the railway line has disturbed the foundations or something but it's a bit of a mess over there. The people in the shops just behind us were all told to get out real fast and we're just waiting to see if the rest of it is going to come down."

Andrew Proudlock, Waikato Demolition director, says they were aware that the building was old and vulnerable and the collapse was "just one of those things".

"These buildings have a mind of their own, they're all old, this one has a really weak earthquake strength test and obviously it's been windy today. The guys were removing part of the roof and there was a large gust of wind and it's toppled part of the wall basically and they saw it and they moved safely out of the way when it caved in."

Mr Proudlock says he's just glad that none of his staff got hurt.

"They're all a pretty hardy bunch, they'll be fine, a couple of beers tonight and they'll be okay ... it's just one of those things and we're addressing it. We just got to make [building] safe now and we'll probably have to change our methodology for bringing it down in the future."

He says they thought the exterior block walls were strong enough to support themselves before taking part of the roof away.

"But obviously it hasn't been so we will be pulling down the facade and the verandahs first before we do anything else from now on."

Eight staff are currently working on the site and three were working near the area of the collapse, he says.

Mr Proudlock says they will file an incident report with WorkSafe NZ.

"It's already been certified and we've got a demolition permit ... we know it's weak and it's very dangerous dismantling it and we're doing it piece by piece. There's no machines in there it's all been done by hand and we haven't touched the outside walls and it's just gone boom and come down with the wind."