The advice was issued for greater Sydney – where the temperature could hit 37 degrees in the west – the greater Hunter, Far South Coast, North West and Upper Central West Plains.
Residents in the affected areas were urged to action their bushfire plans and pay attention to fire and weather reports.
The warning came on a day in which a grass fire outside of Mudgee, about 200 kilometres north-west of Sydney, threatened homes, triggering warnings for some residents to leave.
The grass fire was later downgraded to a watch and act, even as the blaze remained out of control, having burned more than 330 hectares.
Firefighters on the ground and in water bombers were working to hold the blaze near St Fillans and Home Rule but shortly before 6pm said the threat to homes had eased.
Residents were urged to keep monitoring the situation.
A total fire ban will be in place on Tuesday in the the Greater Sydney, Greater Hunter, Far South Coast, Northern Slopes, North Western, Upper Central West Plains, Lower Central West Plains and Central Ranges regions.
Some residents had earlier been told it was too late to leave under an emergency warning which ran from about 2.30pm until 5.30pm, while others had been told to head towards the town of Mudgee.
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“An emergency warning is in place for the Home Rule, St Fillans, Eurunderee and Budgee Budgee areas due to a fast-running grass fire that is burning in the vicinity of Henry Lawson Drive and Drews Lane in a southerly direction towards Lowes Peak,” the Rural Fire Service said about 4.30pm.
“If you are in the area of St Fillans, you are in danger. It is now too late to leave. Seek shelter now inside a solid structure such as a house.
“If you are in the area of Eurunderee and Budgee Budgee leave now if not prepared towards Mudgee.”
A little under two hours after the emergency warning was first issued, the blaze crossed Henry Lawson Drive.
It is believed there are around 20-30 homes in the area.
Residents have been warned embers could make the fire spread quickly and unexpectedly.
“Embers will be blown long distances ahead of the main fire front starting spot fires, these may impact your home earlier than the main fire front,” the RFS said.