Concrete investigators say their scanning shows many buildings have not been constructed according to the plans.

They were "astounded" and "appalled", Jane Roach-Gray of Wellington company Concrete Structure Investigations said.

It is using ultrasound technology developed with Crown agency Callaghan Innovation, to look up to two metres inside concrete columns, beams, walls and floors.

The technology is "pioneering", Dr Paul Harris of Callaghan Innovation said. They also use ferro-scanning and ground-penetrating radar.

Critical structural parts were defective or missing in 1100 of the 1200 buildings they had scanned since 2016, Ms Roach-Gray said.

"The divide occurs between what's in the plans and what ends up in the structure," she said.

"Some key structural elements are not going in correctly or they're not going in at all, and of course, once they're covered up with concrete, we - any of us - don't know what's actually gone into the building."

Her partner Michael Roach-Gray has four decades experience managing construction sites. He thought he had seen it all, until he began looking inside concrete.

He said to call it a "crisis" was to understate it - "it's bullshit", he said bluntly.

"Cover it in concrete, nobody will know," he said.

"And when the structural elements aren't there, like they should be, like they're on the drawings, and they're sitting in the waste bin at the skip, they just say, 'Oh, nobody will know'."

The weakened structures include apartment blocks, offices, public building, most constructed since the 1980s, and some new or still-to-be-finished buildings, including in Auckland, Hamilton, Tauranga, Hawke's Bay, Wellington and Christchurch.

The buildings cannot be identified for legal reasons.

The investigators were aware of legal action being taken, but in each case they knew of, it had been settled out of court - keeping the problems out of the public eye.

Commonly, consulting engineers find a flaw, and call in scanners, who find many other faults. In other cases, a contractor closes down, and the new builder who is brought in asks for scans, to be sure of what they are dealing with.

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