Sydney's tsunami threat

An underwater landslide on the continental shelf could send a tsunami to Sydney that would swamp the low-lying suburb of Manly and damage more than 1000 buildings, a study has found.

The Tsunami Research Centre at the University of NSW found that a five-metre tsunami was the ''most likely worst case scenario'' and would send a wave three to four metres high down The Corso.

The author of the study, Dale Dominey-Howes, said it was ''likely'' that a piece of the continental shelf would one day break off and cause a tsunami as water rushed in to fill the hole, but he could not say when that might happen.

''Eventually an event like that will occur ... but the probability is unknown.''

While a ''moderately large continental landslide'' would be far less powerful and destructive than the 2004 tsunami that devastated parts of South-East Asia, the study finds it would cause massive damage in Manly's low-lying districts.

''If it happened on a Saturday in December you would expect catastrophic loss of life,'' Dr Dominey-Howes said.

The study looks at the vulnerability of buildings and estimates 169 hectares of land would be flooded up to seven metres deep in the lagoon and 1133 buildings would be damaged or destroyed.

The study, conducted in partnership with the Sydney Coastal Councils Group, also examines the likely impact of such a tsunami hitting Maroubra.

Because of its higher elevation, only 27 hectares would be flooded there, with a maximum depth of three metres and only 96 structures affected.

Dr Dominey-Howes said such an event would ''prevent significant challenges to state emergency services''.

He said the study, which identifies the vulnerability of each building, allows councils to better plan for vulnerable areas and ensure suitable design.

The study colour-codes buildings at risk. Those marked orange and red are likely to be ''complete write-offs'', Dr Dominey Howes said.

The general manager of Manly Council, Henry Wong, said Manly was 3.5 metres above sea level and especially vulnerable.

''We have referred the issue of a tsunami to the local emergency management committee and asked them to model for a tsunami of up to five metres,'' Mr Wong said.

He said the council had received advice that residents in 2500 houses would have five to 20 minutes from the time of a submarine landslide before the tsunami hit the beach. The council had to take warnings seriously.