Concrete investigators are calling for mandatory independent quality assurance and pricing controls in an industry increasingly under pressure.
New imaging technology has revealed hundreds of major buildings nationwide have defective or missing concrete or reinforcing steel.
Concrete investigators say their scanning shows many buildings have not been constructed according to the plans.
Engineers are also making mistakes during construction, they say.
Auckland engineer Gordon Hughes says more than half of the structures that he's reviewed since inquiries began in 2003 have had "multiple, significant and serious mistakes".
"Many of the mistakes are the same or similar, even though they've been designed by different engineers, from different firms."
Despite comments made by Engineering New Zealand and implied in a recent paper in a journal from the Structural Engineering Society, SESOC, these were not small or sole-practice engineers, he said.
"All were engineers in mid-size and larger practices, including at least one from a multinational firm."
The engineers, and builders, are being thrown together on complex jobs where lowball quoting meant profit margins can be less even than two percent of the total quote.
Jane Roach-Gray of Wellington company Concrete Structure Investigations said this is why there was not the time, skill or willingness to do a job right or remedy a botch-up.
"Busy consulting engineers, they're called up - even with best endeavours - at the last minute," she said.
"Constructions under pressure - they [contractors] will say, 'The concrete trucks waiting, you need to get here and check that these drag bars are gone in and they're connected'."
Engineers "end up turning up and not seeing everything that they need to see, or being told that things are being put in correctly and only seeing a small margin of them".
Junior engineers or engineers not familiar with the job might be called on to sign off the steel reinforcing, having checked work done by subcontractors operating far down the food chain where accountability was diluted, she said.
"If you're working for a company that's always on the red line and trying to get things done in half the time, you're not going to have that layer of administration. There's very little lead-in time, very little planning."
This culminated on-site in a lack of rigorous, independent checks, of the sort that councils try to provide.
The system was broken, though whether there was any actual fraud taking place, was not for them to comment on, Ms Roach-Gray said.
Her partner Michael Roach-Gray said those contractors doing good work were canny and performing in spite of the huge pressures, while others were unscrupulous or incompetent, or just frazzled.
It was getting worse, he said, as trained staff became harder to find.
Firms were collapsing, leaving a legacy of bad builds behind, he said.
"Essentially the ones that do go broke, especially in the later years, they are the ones that actually are constructing a really poor construction, because they are really under pressure.
"That just exacerbates the poor skillset on site, and it's get it up at all costs."
The stakes were higher still in high-quake areas, because earthquake assessments (to arrive at New Building Standard, or NBS, scores) were being done presuming the building plans were correct, he added.
The Roach-Grays said they were speaking up to call for urgent industry change, which must centre on the government legislating to make it mandatory to use independent quality assurance and to set up pricing controls.
"Nobody wants to hear about price fixing," Ms Roach-Gray said.
"But there are some standards that construction needs to jump over so that they can earn a profit. So we need to regulate it, because if it's unregulated, the race to the bottom just continues."
The government and its building officials have been asked to comment on the findings.