Despite major flooding in the NSW Central West town of Forbes last week, this week's rainfall is "not expected to aggravate that flooding".

"The flood peak has moved past Forbes," she said. "It does take quite a bit of time for the water to move through the river system."

About 1800 residents from 800 households were ordered to evacuate on Tuesday when the Lachlan River approached major flood levels, although the risk eased by Wednesday evening.

Sydney's prolonged damp spell is leaving weather gurus anxious about heightened flood risks ahead of summer as dams are already full.

The city's main water catchment, Warragamba Dam, is 99.2 per cent full heading into what's predicted to be a wetter than average summer.

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This time last year, Warragamba Dam was 96.4 per cent full. A few months later, the dam overflowed during the major flooding in March in the Hawkesbury-Nepean region in western and north-western Sydney.

"What this rain is doing is keeping catchments wet across much of the state, Sydney included. Water storages are full, it’s speaking to that increased flood risk this severe weather season," Ms Imielska said.

"With saturated soils and storages being full, there's no capacity for rivers and the broader environment to soak up that rainfall. It just means we don’t need as much rain for flooding to occur."