First there was drought – now comes the flooding rains.

From the Pilbara to Paddington, torrential downpours are dousing bushfires, soaking parched pastures and turning formerly dry watercourses into torrents.

And giving emergency officials a headache.

As Cyclone Damian crossed the coast of northern Western Australia on Saturday, bringing 200 km/hr winds and driving rain, towns between Port Hedland to Onslow were battening down as the big blow approached.

Heavy rain is also expected to cause flooding in the Pilbara and Gascoyne regions, with falls of up to 300mm or more possible in some areas.

Karratha had already experienced 109mm of rainfall by about 6:30pm AWST on Saturday, with the town in lockdown in anticipation of worse to come.

The skies open – and flood fears rise – from WA to NSW_1

Across the country in NSW, rain continued to bucket down, with residents across Sydney, the Illawarra and the South Coast urged to prepare themselves.

The Bureau of Meteorology expects widespread rainfall to continue on the state's coast, after some stations in northern NSW recorded more than 300mm within 48 hours.

The BOM acting NSW state manager Jane Golding said rain was expected to intensify overnight and through Sunday.

A severe weather warning for very heavy rain, high winds, large waves and tides has been issued along the entire coast from the Northern Rivers region to the South Coast.

"Potentially we haven't seen anything like this since the late 1990s," Ms Golding said.

It drew swift condemnation from NSW Police minister David Elliott, who labelled the driver a "boofhead" and asked police to investigate.

BOM says the trough bringing persistent rain and windy conditions will continue extending into the southern coast on Sunday.

Damaging wind gusts exceeding 90km/h are possible along the coastal fringe between Ulladulla and Forster, and damaging surf conditions are forecast to develop on Saturday night or Sunday.

BOM also warned that the burned-out landscape and loss of vegetation on the fire-affected South Coast had left it vulnerable to landslips.

There were 38 fires still burning throughout the state on Saturday evening.

-with AAP