Oyster farmer Mark Ralston says under strict health regulations the river is closed whenever more than 30mm of rain is received. On Saturday between 75mm and 90mm fell in different areas around the river.

"The storm hasn't done any damage, it's the fresh runoff," Mr Ralston said. "It will be good [in the longer term],it will flush a bit of food into the river. Coming on at this time of the year, it will be a worry when we reopen again."

Mr Ralston said testing of water samples took two days, and he expected harvesting would not resume until later in the week. Farmers had anticipated the rain and had enough oysters in store to maintain supplies until then.

"I hope we can open before Christmas. We like to have fresh oysters," Mr Ralston said.

Meanwhile, grain and cherry growers harvesting their crops in the Young and Harden districts have so far dodged the worst of torrential storms, but rain forecast for this week could spoil wheat.

A major grain producer in Harden, Peter O'Connor, said his family's crops had escaped storms that had hit the Canberra district over the weekend.

"We could have got a lot more rain out of these storms than we did, we felt like they were going north and south of us actually," Mr O'Connor said.

"We've had between 33 and 50mm for the week and we are yet to find out what sort of damage that has done to the wheat. We are probably only about a third of the way through our wheat harvest.

"The worry is how much humidity was around, crops stayed wet for two or three days. I'm doubtful the moisture will be low enough [to re-start harvesting]."

The O'Connors' canola had been harvested with a better result than expected.

"The wheat is the worry, there is more rain forecast for Wednesday and Thursday, following a dry spring which robbed crops of their full yield potential," Mr O'Connor said.

"Most crops have been average to above average," he said.

Consistent rain can cause cherries to absorb too much water and split. But Young cherrygrower Scott Coupland said trees had dried off quickly after storms. "Being as dry as it is, there is a bit of give in the cherry and the tree. It's been more beneficial than harm."

NSW Department of Agriculture spokesman Peter Matthews said the southern area was the last part of NSW to complete harvesting of winter crops.

"The rain over the last seven to 10 days has delayed harvest, but because they were storms, some parts have been able to get going [with harvesting] again," said Mr Matthews, a technical specialist for winter cereals.

"We will see a little bit of crop weather damage, there have been some fairly heavy storms, so we might see some wheat that's sludged down, or we might see some minor shelling because of hailstorms that have come through."

Mr Matthews said the wheat was just ripening when rain arrived, so the risk of it shooting before harvest was lower. If rain persisted the risk would increase.

Department of Agriculture forecasters expect the total Australian winter crop to fall by 16 per cent, but that is still 4 per cent higher than the average of 35.7 million tonnes in the 10 years to 2012-13.

Harvesting is completed in Queensland and northern NSW.