Severe thunderstorms, flooding and heavy rainfall impacting inland northern NSW areas are expected to move towards the coast on Sunday, the Bureau of Meteorology said.
The wet weather is being fuelled by a low-pressure system working as a "magnet" along the east coast.
"This system will act as a magnet - it will act as a sink for moisture and strong winds pulling in from out over the ocean and being directed against the NSW coast," senior meteorologist Miriam Bradbury said.
Slow-moving thunderstorms in the northern inland parts of NSW are forecast to move east, spilling into southern Queensland, and could deliver local heavy rainfall, leading to flash flooding.
Totals between 70mm and 100mm are possible.
The low-pressure system will deepen off the coast on Sunday, Ms Bradbury said, dragging in significant moisture and stronger winds.
Areas of the Sydney, Hunter and mid-north coasts should expect heavy rainfall on Sunday, with strong and gusty winds generating dangerous surf off the coast.
The highest totals are forecast for the Northern Rivers, the Hunter region, the Central Coast and the Blue Mountains.
Long weekend travellers have been warned flash flooding caused by widespread rain could impact journeys.
"It is worth taking the time to plan out a couple of alternative routes in case you find the roads cut off due to that flash flooding," Ms Bradbury said.
Emergency services have issued a stay informed warning for Tumbulgum, in far north NSW, and surrounds following the bureau's forecast.
Minor flooding could occur along the Tweed River on Saturday at high tide.
19°C