The High Court has been told poor workmanship and badly designed dwellings is responsible for leaky homes, not the cladding material.
Legal action against the Australian building products manufacturer, James Hardie, is being taken by 144 homeowners of 151 Harditex clad homes.
They claim their houses' lack of weathertightness is due to the Harditex cladding.
Justice Simon France heard today from the lawyers representing the company, who laid the blame of leaky homes on builders and architects.
The hearing, which began last week, comes after the initial claim was filed in 2015.
Eight properties have been put forward by the plaintiffs as evidence for the judge to decide whether James Hardie bears ultimate responsibility for the leaky homes.
Last week, the judge heard from the plaintiffs who alleged the company was negligent and made misleading statements in relation to the manufacture and sale of Harditex, which was sold between 1987 and 2005.
The homeowners' lawyer, James Farmer QC, told the court no testing was done to back up James Hardies' claim that the product would last for 50 years or more.
He rejected James Hardies' argument that poor installation was to blame for the cladding's failures.
Farmer said the product was inherently defective.
This morning, making their opening statements, the defendant's lawyer Jack Hodder QC said the cladding was a well designed and durable product, and many experts would contest such allegations.
Hodder said bad house design and poor installation of the cladding by builders caused the leaky homes.
He said the case law being applied to this hearing related to territorial authorities or builders, of which James Hardie was neither.
The hearing is expected to last 16 weeks in total.