A powerful storm cell has sparked a weather alert for heavy rain and flash flooding in parts of NSW as the city braces for a wet and cold New Year's Eve.

A severe weather warning for torrential rain that may lead to flash flooding has been issued by the Bureau of Meteorology for Armidale, Inverell, Moree, Narrabri, Mungindi and Bingara.

The warning comes after persistently wet conditions throughout NSW on Wednesday, with the heaviest falls recorded in Crinolyn west of Moree, where 55 millimetres pelted down in just two hours.

The Northern Beaches had the heaviest falls in Sydney, with 105mm bearing down in 24 hours in Terry Hills and 63mm in Manly.

Bureau of Meteorology duty forecaster Gabrielle Woodhouse said the grey conditions are driven by a moist air mass coupled with a low pressure trough.

"What we're seeing on the radar is a number of showers in the east while through the north west slopes and plains and tablelands we have some storms," she said.

Earlier today, several roads were closed due to flooding across the city including at Rozelle and on the northern beaches and Central Coast.

Flooding continues closed the Wakehurst Parkway in both directions between Narrabeen and Oxford Falls.

At Terrey Hills, Mona Vale Rd was closed westbound and in Gosford, the Central Coast Highway had one lane closed.

The miserable conditions are set to persist for New Year's Eve, with showers expected in Sydney, and the possibility of storms in other parts of the state.

"We could see some more severe storms inland NSW west of Dubbo.

"In Sydney for later in the evening there will be quite cloudy conditions and the chance of a shower or two. Temperatures will be quite mild with a maximum of 24.

In the Sydney metro, wet weather will continue for most of the day before a possible easing in the early evening.

The State Emergency Service said overnight crews responded to several calls relating to trees and branches down, as well as leaking roofs.

"If we do get weather warnings issued today there is definitely potential for damage to properties and impacts on the communities," a spokeswoman said.

Originally published as Flood warnings as storm cell hits NSW ahead of NYE