A severe weather warning for heavy rainfall in Sydney and NSW’s Illawarra and Central Tablelands districts has been issued today by the Bureau of Meteorology, which says flooding is possible for numerous river catchments, including the Hawkesbury and Nepean rivers.

The Bureau forecasts “significant” rain this weekend and into next week. The heaviest rainfall is expected between Port Stephens and Batemans Bay, including Sydney.

With catchments already saturated and dams at or near capacity, the Bureau says there is heightened risk of riverine and flash flooding from Saturday, as well as landslips along central parts of the NSW coast. A Flood Watch is active for large parts of the NSW coast, with some possible major flooding forecast.

"Widespread persistent heavy rain is likely to develop along the central NSW coast on Saturday and continue next week as a coastal trough develops and then deepens along the coast,” the Bureau said today.

“There is a potential for an East Coast Low to form within the trough early on Monday and prolong the rainfall for several days.”

River levels are expected to rise from tomorrow, with the largest impacts starting Monday or Tuesday. Wind and waves will also increase along parts of the coast tomorrow, with a risk of coastal erosion for some areas.

Longer-range forecasts indicate the heavy rain over eastern NSW could continue into the second week of July.

Further north, a large band of rain extends from Darwin into south-east Queensland bringing widespread unseasonal rain to much of the northern regions of the NT and western and central Queensland today.

The Bureau says this system will move towards eastern Queensland tomorrow and likely ease on Sunday, though it is expected to redevelop along the east coast of Queensland on Monday bringing more rainfall.

The warning comes after the insured loss estimate from floods that struck NSW and Queensland in February/March jumped to $4.8 billion, to be Australia’s costliest weather event in two decades.

It is only surpassed by the 1999 Sydney hailstorm’s $5.57 billion losses, and 1974’s Cyclone Tracy - the only other Australian weather event with insured losses above $5 billion, in normalised terms.

Insurers are already battling close to 225,000 flood-related claims (around 30% have been closed and $1.5 billion paid to policyholders so far), with NRMA Insurance saying it alone received 22,405 home claims in NSW in March-May for damage caused by wild weather - 19,170 for storm and 3,153 for flood.

Communities along the east coast of NSW and Queensland - especially those living on or near any rivers, creeks or streams – and Australians travelling nearby these school holidays are advised to enable push notifications on the BOM Weather app for the latest advice.

The timing, amount and location of wind, rainfall and riverine flooding impacts will be driven by the evolution of this weather system, with a “degree of uncertainty remaining,” the Bureau said.

Further updates to forecasts and warnings will be issued over coming days.