While the likelihood of damaging floods, storm surges and king tides will increase around the coast, Sydney can expect to see ''extreme events'' once a month by 2100, the commission's report The Critical Decade says.

''While a sea-level rise of 0.5 metre … may not seem like a matter for much concern, such modest levels of sea-level rise can lead to unexpectedly large increases in the frequency of extreme high sea-level events,'' it said.

Confirmed sea-level rise around Australia's coast amounts to a few millimetres per year since the early 1990s, with the highest levels of about eight millimetres recorded off Western Australia and the far north.

Off Sydney, the rise has been much more moderate, at an average of just 1.8 millimetres a year - not noticeable to unaided observation. However, the report notes, even tiny amounts of sea-level rise can have a big impact on king tides, storm surges and flooding.

''The observed sea-level rise of about 20 centimetres from 1880 to 2000 should already have led to an increase in the incidence of extreme sea-level events,'' the report said.

The Climate Change Department says a moderate sea-level rise this century will cost $63 billion in terms of changes to infrastructure and housing locations.

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