Leaky homes lawsuit: James Hardie says complainers a drop in the ocean when vast number of good homes taken into account

A company getting sued in a major leaky homes class action says poor workmanship is to blame for problems in New Zealand houses.

And the defence team for James Hardie said company experts hadn't even been allowed to access some of the allegedly leaky buildings in question.

Hundreds of homeowners have blamed the company's Harditex monolithic cladding for expensive, unhealthy problems including damp, mould and rot.

The plaintiffs - the people suing the James Hardie group of companies - are seeking about $220 million in damages.

At the High Court in Auckland today, James Hardie defence counsel Bruce Scott said some people refused to have invasive testing on properties where Harditex was installed.

"At each of the other five sample properties, the defence experts have either had not any access at all ... and nor have the plaintiff's experts."

The company has argued moisture issues at roofs, window units and other places resulted from failure to follow the James Hardie system's requirements.

"The errors we're seeing are errors we believe not any competent builder would have made," Scott added.

The company's lawyers also told the court Harditex was subjected to stringent tests in some extreme conditions.

Justice Christian Whata was told some sheets were submerged in water at 50 degrees Celsius for 350 days.

Other samples were buried for six months.

Defence counsel John McKay said testing pushed products far beyond real-world conditions.

He said Harditex had a history of good performance and maintained its appraisal from independent research organisation Branz, which approved the use of Harditex.

"We know it fails in extreme conditions," Justice Whata said. "The issue is, does it fail in more moderate conditions like that of New Zealand?"

"What matters is whether you've managed to restrict entry to incidental moisture ingress," McKay said.

He reiterated the company's assertion that of tens of thousands of New Zealand homes clad with Harditex, only a tiny minority sparked complaints.

"Such a failure rate is inconsistent with the plaintiffs' assertions about the fitness of the product."

Harditex was withdrawn in 2005. The court heard James Hardie enacted an "improvement plan" leading to a new product named Monotek.

But McKay told the court: "Improving something does not mean the predecessor was defective."

Jack Hodder QC, also representing the group of companies, said it was "nonsense" to suggest James Hardie enabled a system where anyone was expected to install Harditex.

"The assumption is there will be good building practices deployed in putting these walls together."

The company has said a deregulated building environment from the late 1980s and shoddy building practices were to blame for leaks and defects.

Hodder said the lawsuit included "attempts to ensnare" James Hardie Industries PLC, the group parent company, now domiciled in Ireland.

The homeowners earlier this week argued James Hardie was well aware of Harditex vulnerabilities but failed to alert the public.

The plaintiffs have also said the Harditex system was inherently flawed because it required an impossibly high level of workmanship not to leak.

And lawyers for the homeowners told the court meaningful testing for Harditex in New Zealand conditions was minimal or non-existent.

The class-action hearing is expected to last until at least late September.