Tens of thousands of NSW residents are under evacuation orders today after 100mm of rain swamped many catchments, sending rivers and dams beyond containment lines – with more wet weather still to come.

For Sydney, rain is likely to continue this afternoon before easing overnight, though the system is then set to move north to the Central Coast and Hunter region.

The Insurance Council of Australia (ICA) told insuranceNEWS.com.au today that it is still assessing the situation and awaiting clearer information.

The deluge comes after the insured loss estimate from floods that struck NSW and Queensland in February/March jumped to $4.8 billion, making them Australia’s costliest weather event in two decades – only surpassed by the 1999 Sydney hailstorm and 1974’s Cyclone Tracy in normalised terms.

Insurers are already dealing with around 225,000 flood-related claims from earlier events. About 30% have been closed and $1.5 billion paid to policyholders so far.

“We have another at least 24 hours to go before we hear that the rains may subside,” NSW SES Commissioner Carlene York said. “The ground is saturated and the rivers are fast-flowing. The dams are overflowing, so it is particularly dangerous out there, particularly in the Hawkesbury-Nepean.”

The Bureau of Meteorology (BOM) says a number of locations from Newcastle to the Illawarra received more than 100mm of rainfall in the past 24 hours, and continued downpours are expected along the NSW coast for the remainder of the week.

Warragamba Dam in Sydney's southwest has been spilling since the early hours of Sunday morning, and the Australian Defence Force has stepped in with manpower and aircraft to assist. Seven evacuation centres have been established to accommodate the 32,000 Australians affected by evacuation orders. Tere has been one fatality reported so far – a man who died when his kayak overturned.

"For many communities, this is the fourth flood they have seen in less than 18 months,” NSW Minister for Emergency Services and Resilience and Minister for Flood Recovery Steph Cooke said.

"Some of those images are truly heartbreaking – seeing people's homes [and] lives turned upside-down again, and their livelihoods very much impacted.”

BOM Meteorologist Jane Golding says the accumulation of rainfall for some locations is expected to be “very high” again today.

“The catchments are not drying out and so any follow-up systems that we get over the next couple of weeks – we are still in east-coast-low season – means the flood risk does remain.”

The numbers were “comparable to what we saw in March,” Ms Golding says. But rather than occurring over seven days, “this time we are seeing really high rainfall rates on individual days".

The bureau expects major riverine flooding to begin in Windsor this afternoon before reaching Wiseman’s Ferry tomorrow, with flash floods also possible.

“The flood levels are high, the water is flowing really quickly. It is dangerous out on the rivers, and we do have some more rain to fall which means the flash-flood risk is not over yet either,” Ms Golding said.

Also keeping emergency services busy today was a Hong Kong-registered bulk carrier with 21 crew that left Wollongong this morning and lost power. Tugboats were being sent to help it out to sea.

“It has been very difficult time for many months,” NSW Premier Dominic Perrottet said. “To have this flood event off the back of others makes it more challenging.”

He says there have been 116 flood rescues in recent days, and 1593 requests for assistance.

Blacktown Mayor Tony Bleasdale OAM says major policy and infrastructure changes are needed if population growth is to continue in areas like his in Sydney's west.

"We can't continue to build if in fact we're risking people's lives … We just cannot continue to build in flood prone areas unless we put in place a whole range of mitigation policies that can really reduce the risk because at the end of the day, we need to be able to evacuate people from these areas," he told ABC NewsRadio's Thomas Oriti.