Firefighters across NSW have battled through the hottest part of the day with a fire-generated thunderstorm in the Northern Rivers threatening to escalate an already out-of-control blaze's spread.
The thunderstorm formed over a fire in Nymboida, 30km southwest of Grafton, about 5pm on Wednesday.
"As a result, there is a potential for escalation in fire behaviour, with erratic winds, dry lightning and significantly faster fire spread," the NSW Rural Fire Service said.
An emergency warning has been issued for the area with residents advised it is too late to leave and to seek shelter.
High to extreme fire danger ratings are forecast for much of Australia this week with temperatures in parts of multiple states expected to be several degrees above the October averages.
More than 900 NSW Rural Fire Service personnel were deployed to battle 72 bush or grass fires burning across the state, with 23 blazes yet to be contained as of Wednesday afternoon.
A total fire ban was in place for six regions in the state's north and northwest with sweltering temperatures in the mid-to-high 30s in some areas.
Extreme fire danger warnings were issued for the greater Hunter, northern slopes and northwestern regions, while those and three neighbouring areas are subject to total fire bans.
Hot and dry conditions with fresh northeast to northwesterly winds were leading to dangerous weather for fires, the Bureau of Meteorology said.
Conditions could be exacerbated by a fresh and gusty south-to-southwesterly change moving through during the day and thunderstorms, which were likely to develop across the region.
RFS operational officer Emilie Koek told AAP forecast thunderstorms could have the potential to spark new fires and erratic winds would cause challenges for firefighters.
She said the public should report unattended fires immediately to ensure crews could attend as soon as possible to contain them.
Grafton in the Northern Rivers region hit 39C on Wednesday afternoon, with fire activity increasing in the heat across multiple blazes.
Keenly watched fires include a 1400ha blaze on Thunderbolts Way, near Gloucester, a 2000ha fireground at Nymboida and a fire that has scorched 1100ha of national park east of Tenterfield.
Residents in extreme fire danger areas should take action to protect their life and property, including having a bushfire survival plan and knowing what to do should fire threaten.
"There is still elevated fire danger over the coming days," Ms Koek said.
"Potentially over the weekend conditions could ease, but we will have to see how the forecast pans out."
Investigations continue into several bushfires on the NSW mid-north coast that are believed to be deliberately lit.
A fire that started last Tuesday at Belmore River, about 15km east of Kempsey, burnt through nearly 3000ha of bushland in the Hat Head National Park before being brought under control.
Police, including arson unit officers, are investigating after the fire was deemed to be suspicious.
