Auckland geologist's warning to people living in or considering buying cliff houses

Home buyers are being reminded to consider the future risks of properties situated on cliffs following the devastating flood damage seen in Auckland last week.

This comes after a number of coastal properties were hard hit during the flooding last week, with some losing their gardens and sections of property.

Associate Professor of geology at the University of Auckland Martin Brook said for decades, Aucklanders have been building close to the edges of slopes for the best views - "sometimes less than 100m setback".

However, Prof Brook warns of the dangers and said "buildings on weak soil with additional lack of setback distance are major problems."

Prof Brook said Auckland Council's proposed Plan Change 78 limits development within designated coastal hazard areas to avoid further risks to people and property.

However, Prof Brook added this does not cover land development on cliffs away from the coastal strip, so "perhaps other planning instruments may be used or indeed modified".

He said the effects of any future slips are likely to be very site-specific, which means taking into account the local topography (shape of the land), local geology, soil properties and any land use changes, such as inadequate drains, or recent tree cover removal.

Meanwhile, Prof Brook has a message for those considering building on clifftops. 

"Plan for the future and bear in mind the environment and landscape is not static, it changes naturally over time, irrespective of any human-induced climate change," he said. 

He believes a chartered engineer to provide a site assessment and a stability analysis of the cliff would be useful to determine its factor of safety.

"It may help determine if future failure is likely, given the strength of the rock and soil that comprise the slope," he said.

Prof Brook understands that a council ban is unworkable, but believes there are certain things it should be doing to help. 

"Some slopes can be stabilised using a range of engineering approaches such as retaining walls, soil nails, as well as different types of drains, among other approaches," he said.

Real Estate Authority CEO Belinda Moffat urged the importance of vendors disclosing property issues to potential buyers.

"It's important that they [vendors] are honest and upfront with the licensee about these kinds of issues or risks," she said.

Moffat said she expects the licensee to seek out information on a property they are listing by asking the vendor about any known (or potential) defects or damage, as well as being able to identify any potential issues themselves, upon inspection of the property.

"This includes understanding risks arising from floods or subsidence that may be apparent," she said.

Meanwhile, Moffat said that vendors may provide their own geotechnical information or report, but buyers should keep in mind that if a report proves to be inaccurate, they have more direct legal recourse with the provider of a report that they commissioned themselves than with a report that was prepared for the vendor.