Stacy Smith's family were forced to quit their new beachside home after mould rotted the carpet of one of the children's bedrooms and infiltrated the walls and ceiling, resulting in dangerously high levels of mycotoxins.

"My family has a major sensitivity to mould," said Ms Smith, a pseudonym. The family are now suing the builder for costs associated with repairs, remediation and living elsewhere.

Within two months of moving in, Ms Smith said heavy rain sent water running down the walls of the modern home on Sydney's northern beaches. Behind its walls, the rain was pouring in through a leak in the roof which helped spread mould, allegedly contained in damp building materials hidden behind plasterwork.

The wettest March in 42 years has created the "perfect" conditions for mould, said Ibrahim Ech, the principal work health safety consultant with Safety and Environmental Services.