Australia had for the last 7 or more years experienced a serious drough.... And then the destructive bushfires and finally came the rains, as many farmers had predicted.

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The entire NSW coast was drowned by at least 200 millimetres, and in some places, more than 400mm of rain.

Flooding stretched 600 kilometres from Sydney to the Northern Rivers.

The BOM's Victoria Dodd said the state had experienced "the most significant floods … in decades".

"We've seen exceptionally high flood levels, high-velocity flows, lots of dangerous debris in these floodwaters."

NSW has 40 river catchments, half of which flow east to the coast.

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Dr Johnson said an unusual feature of this flood event was that the heavy rain persisted for five days.

"Rainfall of 200mm in one day is rare. It might occur once every 10 or 20 years, but when you look at the five-day total, that's unprecedented," she said.