The demolition of an earthquake-ravaged historic building has begun despite an outcry from heritage campaigners.

Protesters say the decision to knock down the 104-year-old Manchester Courts building in central Christchurch was rushed.

"It's a complete disaster for New Zealand heritage," said Christchurch Civic Trust deputy chairman Neil Roberts.

The seven-storey building was once the tallest in Australasia.

The co-owner of the building, Richard Peebles, said a series of engineering reports had made it plain the building was so severely damaged by the September quake that it had to be demolished before it collapsed.

"Apparently some people believe that the building should be saved even if it does put human life at risk, but I'm not one of those people."

The decision to demolish still caused him "extreme sadness".

"We loved the building with a passion and we spent hundreds of thousands of dollars on it. The opportunity for saving this building and earthquake-strengthening was before the quake, not afterwards. The horse has bolted."

Mr Roberts said Manchester Courts had the same heritage status at Christchurch Cathedral.

"This building is strong - it would have been rubble otherwise. This building is being demolished without thorough investigation - and that's a real problem. It's a loss not only for Christchurch, it's a loss for New Zealand and Australasia."

It will take six to 12 weeks to complete the demolition and several nearby streets have been cordoned off, affecting more than 50 businesses.

A new three-storey building is planned for the site.