An erratic fire in Tarcutta south-western NSW now at "watch and act" sparked a new emergency fire warning on Saturday evening as fire services prepared for a surge in extreme conditions on the South Coast leading up to New Years' Eve.

With temperatures set to rise above 40 degrees in western Sydney and other parts of the state and north westerly winds picking up in coming days, the Rural Fire Service has forecast "severe and extreme fire danger coming into Tuesday".

The Gospers Mountain Fire will begin to impact the township of Glen Davis over the coming days. Credit:Wolter Peeters

A spokeswoman for the RFS said it was concerned about the progress of fires in southern NSW on Sunday and the Shoalhaven area on Monday and Tuesday.

A lull on Saturday was disrupted by late afternoon when a fire on Westbrook Road in Tarcutta was upgraded to "emergency warning" after it burnt through more than 1500 hectares.