Simon Hughes QC, one of the lawyers representing homeowners, said much of the case focused on the allegedly disastrous Harditex cement and wood fibre cladding system.
The main question, Hughes told Justice Christian Whata, was whether Harditex was inherently flawed.
"Does Harditex work as a barrier cladding system? Does it stop the water getting in at the start? We say no."
He said water ingress, mould, and degradation of sheets were common problems.
Hughes told Justice Whata he expected James Hardie to blame "bad builders" for the leaky homes scandal.
But he said different builders and tradespeople worked on hundreds of different properties which now had "a very similar menu of problems".
The product was introduced to New Zealand in 1987 and withdrawn in 2005, he said.
"The troubled history of Harditex is quite extraordinary. The defendants knew they had real problems with Harditex."
Hughes said James Hardie should have ensured its system was weathertight in New Zealand conditions and would not damage plaintiff's properties or health.
'SEEPED' INTO BUILDING PSYCHE
He said Harditex was marketed largely to builders, and James Hardie had an elaborate training programme for tradies.
"It was a soft sell."
The court was told building industry merchants would then recommend Harditex to clients.
Hughes said Harditex and associated brands and products then "seeped into" New Zealand and the industry psyche.
He said James Hardie had a duty to test the system's performance in applicable local conditions.
Hughes said Harditex was tested in damp Queensland conditions in the 1980s and '90s and after five years, samples had completely degraded or disintegrated.
The court heard Harditex found a New Zealand market when previous claddings containing asbestos were phased out.
"There was no relevant ancestral product which could be used as a proxy for performance," Hughes said.
"The removal of asbestos meant degradation in the presence of water was more likely."
Harditex wasn't tested at all for weathertightness and James Hardie might have just assumed its product would be good enough, Hughes told the court.
The class action involved hundreds of properties.
In a 2019 judgment, Justice Whata said the central claim in the proceedings was that the defendants manufactured, supplied and/or promoted defective exterior claddings.
The products were sold under the brand names Harditex, Monotek or Titan.
At the time of that judgment, about 1246 properties were said to have defective product claddings.
Most were residential homes, but the people suing James Hardie at that time said five commercial buildings and five retirement villages were also affected.
Hughes is expected to continue his opening remarks before Justice Whata today and tomorrow.
