Fire crews across much of NSW are taking advantage of milder conditions to conduct critical backburning, as they brace for an impending heatwave and predicted lightning in coming days.

The smoke haze blanketing Sydney is expected to worsen on Monday before temperatures hit the low 40s in parts of the state, including the city's western suburbs, on Tuesday.

RFS firefighters working to protect properties as the North Black Range bushfire threatens properties at Bombay on Friday. Credit:Alex Ellinghausen

An out-of-control bushfire is burning on both sides of Lake Burragorang, while an emergency alert has been issued for residents on the western edge of a blaze near Warragamba Dam on Sunday.

The Rural Fire Service issued the alert shortly after 3pm warning residents of isolated properties around Jaunter Road and Tuglow, south of Oberon, to seek shelter as the fire approaches.