A popular brand of timber product used to build Kiwi homes doesn't meet the strict rules put in place in response to the leaky homes crisis.
That's prompted urgent action from government officials.
The Commerce Commission has ruled J-Frame is not everything it claims to be. J-Frame is a laminated veneer lumber - or LVL - made up of multiple layers of thin wood glued together.
"There isn't a question mark over whether it is a rotting risk. It's a question mark over whether it has been labelled appropriately," Construction Minister Nick Smith says.
Regular timber is saturated with a preservative all the way through the wood.
"The manufacturers of the LVL argue that they can't get the boron treatment all the way through because of the layers of the glue. But that the product is just as serviceable and is resistant to rot."
It means the product shouldn't have the mark H1.2 and is wrongly listed on building consents.
1 NEWS has been told more than 10,000 homes have been built using J-Frame
Dr Smith says they shouldn't worry.
"I don't think there is any risk of councils wanting to revisit any of the buildings that have been constructed with J-Frame."
But the Government has told business ministry officials to get J-Frame samples retested.
And officials are reviewing building standards.
"I am always cautious in this area of timber treatment because it was at the head of that huge issue New Zealand had a decade ago with leaky homes," Dr Smith said.
Labour's Stuart Nash says the Government was too slow to act.
"We do know MBIE found out about this over three years ago. The Minister has been well aware of it. It should not be up to the Commerce Commission uncover this stuff," he said.
J-Frame manufacturer Juken New Zealand wouldn't front for an interview but said in a statement: "J-Frame has been in the market for more than 10 years and the company has had no reports of performance issues in the market."
The company is now "carefully considering" the Commerce Commission's ruling.