Sydney is preparing for yet another deluge, as a deepening low-pressure trough puts residents on alert for flash flooding and landslips.
The Bureau of Meteorology (BOM) warned up to 300 millimetres of rain could fall across NSW in the coming days, intensifying on Thursday then weakening on Friday.
A State Emergency Services spokesman said the news of further rain may be "distressing" to people from flood-affected areas but it was important to prepare.
BOM senior meteorologist Sarah Scully said heavy rainfall falling across the South Coast of NSW today would extend to Greater Sydney and adjacent ranges by tomorrow.
She said rain was likely to increase across the Sydney Metropolitan overnight, with the potential of six hourly rain totals of 60mm to 100mm.
Coastal areas around Sydney will be particularly at risk, with isolated heavy falls of up to 140mm possible.
"Heavy and persistent showers over coming days will increase the risk of flash flooding and landslips over already saturated catchments," Ms Scully said.
"Severe thunderstorms are possible at the north-east of NSW on Friday."
A severe weather warning has been issued covering almost 400 kilometres of the NSW coast, from Gosford in the north to Bega in the south, and inland as far as Oberon.
Bushwalkers are being urged to exercise caution, with the intense rain heightening the risk of landslips.
Drivers are also being warned to be cautious.
It comes as the Blue Mountains National Park remains closed as rangers review safety after a landslip at Wentworth Falls that killed a father and son earlier this week.
Most walking tracks will be cordoned off apart from Evans Lookout and Govett's Leap Lookout.
Flood watches have been issued for central and southern coastal rivers, as well as the Macquarie and Queanbeyan Rivers.
Residents in Windsor are preparing for further rivers rises, with minor to moderate flooding possible on the Hawkesbury and Nepean rivers in the coming days.
Ms Scully said conditions would gradually ease by the weekend.
