A flood watch is current for all coastal rivers from the Queensland border to Coffs Harbour.
The Weather Bureau is warning ex-Tropical Cyclone Oswald could affect the north coast in the coming days.
The low pressure system currently off the Queensland coast is moving slowly south.
The Bureau's duty forecaster Jake Phillips says it's expected to bring heavy rain and flash-flooding to the region from Sunday.
He says residents living in river valleys and low lying coastal areas should be prepared.
"It looks like the rain is going to ramp up near the Queensland border later tomorrow," he said.
"Then it's expected to move down to the mid north coast Saturday night through Sunday and Monday as well.
'The current indications are that it has potential to be quite a serious event.
"There's some indications that widespread rainfalls in excess of 100 mm, some spots are likely to get 200-300 mm."
© ABC 2013
15:20 EST Heavy rain in the past few days led the Rural Fire Service to reassess plans to bring forward the bushfire danger period in the Hunter region to September the first.