MUCH of the NSW coast can expect its heaviest rain so far this year this week - and possible flooding.

Weather Channel senior meteorologist Tom Saunders said the state's north-east was already experiencing the start of the event, triggered by onshore winds from the Tasman Sea driving moisture into a trough developing near the coast.

"The trough will slip south and deepen on Tuesday, causing heavy rain from the Queensland border all the way to the South Coast," Mr Saunders said.

"This weather pattern will then remain relatively unchanged through the week with heavy rain continuing until at least Saturday.

"Rain will also spread to the NSW ranges at times and showers will reach the western slopes."

He said "almost the entire" NSW coast could see more than 100mm of rain before the weekend.

"The wettest part of the week will be from Tuesday to Thursday when rainfall may exceed 100mm in a 24-hour period.

"On the western side of the ranges, rain this week will accumulate to around 10 to 30mm.

"The rain should cause rapid but short-lived flooding along coastal rivers and streams, but flash flooding is a greater threat this week.

"Particularly in built up urban areas like Sydney where drainage systems struggle to cope with short bursts of heavy rain."

Rain is expected to ease toward the end of the week as onshore winds and the trough weaken, which should produce a fine weekend.