Pubs and cafes a stone's throw from Windsor Bridge are still open, but supermarket shelves are being emptied by fears of another big flood on the Hawkesbury River in the historic farming region northwest of Sydney.

Bridges in Windsor and Richmond - the only links to some of the region's bustling towns - will be closed if Warragamba Dam overflows and water spills downstream to Penrith and the Hawkesbury region.

With no break in the rain, locals are watching the Hawkesbury rise and recalling the ferocious floods of the past that destroyed roads, homes and schools, and claimed lives.

"Bread was gone (by Wednesday afternoon) because people are worried about the flood and the closure of the bridge," a shop assistant at Woolworths in Windsor said.

Hectares of turf and vegetable farms along Freemans Reach Road, a few hundred metres from Windsor Bridge, are already sodden from relentless rain.

Luxury houseboats and floating restaurants have deserted Windsor's wharf for safer moorings

Further upstream, at Wisemans Ferry, prawn fishers have stopped work due to safety fears.

"We're watching the tide, and some of the fishermen have started to come back home," says Mary Howard, 66, whose family has been fishing on the Hawkesbury for as long as she can remember.

"We're lucky we've built our house above the ... flood zone, but we're still watching closely.

"We have two boats and a pontoon that can withstand high tide.

"We are not yet threatened, but the nuisance of this flooding would mean we can't go to shop and our roads will be cut off."

The Playground Weekender, an annual big-ticket music festival scheduled to be held from Friday through Sunday at Wiseman Ferry's Del Rio Resort, was cancelled on Thursday.

About a dozen music fans in Wisemans Ferry for the festival are being billeted at the resort.

Resort manager Bunny Roberts says no one has checked out despite early warnings by the SES.

"We have about 16 local residents ... and a number of visitors to look after, so we have emergency contingency plans put in place at this very moment," she said.

The Hawkesbury SES has told caravan park residents along the Hawkesbury River to watch flood levels closely, be ready to move their vans to high ground and keep in contact with fellow park residents and management.

Some locals expressed concern about possible sanitation problems at some caravan parks on the river, saying on-site effluent treatment plants could be damaged by floodwaters.

- AAP