Premier Gladys Berejiklian says residents need to follow the advice of authorities over the next week, with extra diversions and road closures to be installed.

The last state of emergency, which also ran for seven days in mid-November amid "catastrophic" fire risk, was the first implemented since 2013.

The state of emergency allows powers to be transferred from the government to Rural Fire Service commissioner Shane Fitzsimmons, including the ability to control and co-ordinate the allocation of resources and evacuate people from property within a declared area.

Mr Fitzsimmons will also be able to direct any NSW government agency to conduct or refrain from conducting its functions.

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Dangerous fire conditions are threatening large parts of the state as up to 100 bushfires rage and communities prepare for a day of high temperatures.

The RFS has extreme fire danger warnings in place for the Greater Sydney, Illawarra and Shoalhaven and Southern Ranges areas.

Mr Fitzsimmons said on Wednesday fire behaviour would be "erratic and significant" due to volatile winds.

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"The heat will start moving through NSW on Thursday and conditions will worsen on Saturday," he told reporters.

A statewide total fire ban announced on Tuesday will remain in place until at least midnight on Saturday.

The main concern is the huge Gospers Mountain fire northwest of Sydney, which is still burning out of control on multiple fronts in the Wollemi National park.

The fire was a watch and act level on Thursday morning but there was no immediate threat to property.

Read also: Widespread closure of NSW forests

The Bureau of Meteorology expects most of inland NSW will see temperatures in the mid 40s while areas west of the Sydney CBD are expected to top 41C.

Of the fires burning around the state, more than half are uncontained.

Australian Associated Press

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