California was expected to get a break from rain, but forecasters warned mud and rockslides are possible in canyons and steep hills as the ground is saturated after three weeks of rain and snow.

Most of the deaths were caused by drowning from flash floods after of heavy rains disrupted celebrations and forced more than 45,000 people to take shelter in evacuation centres, reports say.

The Civil Protection Authority has reported nearly 1,500 incidents across Portugal, mainly in Lisbon and the districts of Santarem, Portalegre and Evora, including flooded streets, tunnels, train stations and shops.

With a wild weather system expected to dump heavy rains across a wide swathe of Australia's east, authorities warned Sydney residents to watch for flash flooding and stay away from flooded roads.

The downpours have dangerously swelled waterways in the low-lying Pearl River basin in recent days, threatening manufacturing, shipping and logistics operations.

Several towns in northern New South Wales already reeling after record floods over a month ago were pounded by an intense low-pressure system overnight.

The death toll from the deluge rose to 18 after a man was found dead in a car swept away in floods in Queensland on Sunday.