Thousands of new homes planned for a flood plain in Sydney's north-west could cost more $6000 a year to insure for flood damage.
The insurance industry has signalled that new developments planned for Riverstone, Colebee, Vineyard and Schofields will remain major flood risks, despite state government plans.
The Hawkesbury-Nepean flood plain cuts through large sections of the North West Growth Centre, which is scheduled to have 70,000 new homes and 200,000 residents.
"Areas such as the Hawkesbury-Nepean flood plain represent potential concentration of extreme flood risk that, if it floods, could be devastating to property owners," an Insurance Council of Australia spokesman said.
"Flood insurance is available for this area, from a large number of insurers, but premiums will typically reflect the level of risk."
The spokesman said insurance companies relied on flood modelling of local councils to assess risk.
Reports to Hawkesbury and Blacktown councils and the state government include much of the North West Growth Centre in the one-in-100-year flood risk.
Schofields mum Sandra Woods told the Sun last month that insurers quoted upwards of $500 a week to cover her family property for floods.
Mrs Woods said her family had risked not insuring the home, despite flood waters regularly inundating their yard.
Developers have already begun marketing land directly behind the family's property for a new residential estate.
Riverstone MP Kevin Conolly all new lots planned for the flood risk zone would be above the one in 100 year zone.
But he noted that parts of properties not above the zone could still be assessed as within the flood risk by insurers.
"There may be cases where parts of a property are below the flood zone but the house is above it," he said.
"That could be a gap in the flood mapping we need to assess."
Mr Conolly said he was in continuing talks with Blacktown Council about flood mapping.
A spokesman for Planning and Infrastructure said the flood zone had been considered and engineered for in the new suburbs.
"To ensure safety, flood evacuation routes are identified and the road network is designed to ensure safe routes out of urban areas in the event of a major flood," he said.