Heavy rain to drench coastal NSW and Sydney: bureau

A weekend of persistent, heavy rain is on the way for NSW as a slow-moving band of wet weather moves across the state's interior.

Showers have persisted for much of the week but are expected to increase on Friday ahead of a dreary weekend, the Bureau of Meteorology warned.

"The rain is still over the interior and the north of NSW ... that will change this weekend though," bureau meteorologist Angus Hines said.

"Into Saturday, we will see this broad area of rain and possible storms move further eastwards."

Clouds started to form in the "border corner" of NSW, Queensland and South Australia on Thursday and are expected to move towards the coast on Friday, Mr Hines said.

Showers were already falling on the coast throughout the week.

The rain is expected to intensify in coastal areas from Wollongong north to the mid-north coast at Port Macquarie, with Sydney and Newcastle also set to be drenched.

"All of these places are in line for a very wet weekend," Mr Hines said.

Coastal districts could receive as much as 150mm of rain, enough to cause flooding, he said.

The bureau is also warning powerful surf conditions could create hazardous conditions for rock fishers and swimmers during the weekend, particularly on south-facing beaches.

The continuing wet weather comes on the back of above average rainfall with Sydney receiving almost 30mm more for April, boosted by 111mm falling on April 5 at the city's Observatory Hill station.