One metre of rain in a week possible: BOM

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A new band of rain is headed for flood-hit NSW and expected to push totals in some places to one metre in the space of just a week.

The Bureau of Meteorology says eastern NSW is in for a double whammy in the days ahead.

A new band of rain moving down from the Kimberley in Western Australia will push through central Australia on Sunday.

On Monday it will pass over outback southern Queensland into northern NSW, where it will link with the slow moving coastal trough that's already caused widespread flooding on the east coast.

BOM meteorologist Jonathan How says it's not yet clear where the rain band will dump the heaviest falls because they'll be generated by hard-to-predict storm activity.

But he says eastern NSW is definitely in the firing line, when the band and the trough come together.

"That's when we'll see a very large burst of rain, across eastern NSW, dipping into northeastern Victoria and southeast Queensland from Monday night into Tuesday," he told AAP.

The first dry day for NSW is not expected until Wednesday.

Mr How said some NSW locations, particularly around Port Macquarie, had already recorded 500-600mm of rain in the previous 48 hours or so.

"By Monday or Tuesday, it's not unreasonable that some places will possibly poking one metre of rain for the whole event," he said of the rain which began in earnest on Wednesday.

BoM experts say the NSW mid north coast is "not out of the shadows" yet, even though rain has eased. More heavy rain in flooded communities is possible on Saturday night, senior climatologist Agata Imielska said.

The trough that's flooded parts of the mid north coast is now moving south and affecting greater Sydney, with the lower Blue Mountains expected to see the worst of the rain in the next 12 to 24 hours.

"For the Greater Sydney region, we are looking at rainfall totals, widespread across the region of around 100mm," Ms Imielska said.

"But really, the heaviest falls are expected over the lower Blue Mountains where we are likely to see 200-300mm. Even within this area of what is persistent and heavy rainfall, we can see local very heavy rainfall, up to two to three times the amounts that might be recorded in nearby areas."

Warragamba Dam, in outer southwestern Sydney, is expected to spill on Saturday afternoon, pushing water into the major river systems around Sydney.

Moderate to minor flooding is possible for Greater Sydney but that will depend on exactly where further rain falls.