Greater Sydney and Greater Hunter are among five areas facing extreme danger from blazes on Tuesday and among eight areas subject to total fire bans.
Tamworth, Dubbo and tourist hotspots of Batemans Bay and Merimbula are among other centres facing increased fire risk.
The warning for elevated fire danger comes as the weather bureau forecasts above-average temperatures and winds becoming strong and gusty along the coast and southern half of the ranges during Tuesday.
Sydney is forecast to surge to a top temperature of 33C
The risk became real on Monday afternoon for communities north of Mudgee, where residents were told it was too late to flee a fast-running grass fire.
Those still in the St Fillans were told to shelter while people further south in Eurunderee and Budgee Budgee were told to travel to Mudgee if unprepared.
Water bombers and firefighters gained control of the blaze late on Monday afternoon, easing the threat to homes.
"If you do not have a plan, decide what you will do if the situation changes," the Rural Fire Service warned.
"Leaving early is your safest option."
All other 107 blazes statewide on Monday evening were burning at advice level.
NSW FIRE DANGER ON TUESDAY
Extreme: Greater Hunter*, Greater Sydney*, Far South Coast*, North Western*, Upper Central West Plains*.
High: Far North Coast, North Coast, Illawarra/Shoalhaven, Monaro Alpine, Southern Ranges, Central Ranges*, New England, Northern Slopes*, Lower Central West Plains*, Northern Riverina, Far Western.
*denotes total fire ban
12°C