The NSW Rural Fire Service (RFS) warns the "bushfire season has arrives" and his declared a total fire ban for parts of the state on Thursday.

Forecast hot and windy conditions have also resulted in a "severe" fire danger rating for Sydney, the Greater Hunter, Illawarra, the south coast, northern Riverena and lower central west plains.

It is the first time the "severe" rating has been used, after the system was overhauled in the wake of February's Victorian bushfire tragedy.

A new top category of "catastrophic" has also been introduced.

It will be used for days when weather conditions exceed the current top level of extreme, when major loss of life and property is deemed likely.

Severe is third in the danger sequence, and when in force, people are advised to leave their homes early.

RFS Commissioner Shane Fitzsimmons says the "severe" warning will be used for the first time on Thursday when temperatures are expected to reach the mid-30s in parts of the state.

"At severe, leaving early is your safest option for your survival. Your home will only offer safety if it and you are well prepared and you can actively defend it during a fire," he said.

"So our advice is very clear to the community. Prepare your home and prepare yourself, by ensuring you and your family knows what to do if a fire does start."

Mr Fitzsimmons said NSW had already seen significant fire activity on the north coast and further inland in the past few days.

"If people need a reminder that bushfire season is here, this is it," he said.

"Now is the time to prepare and make sure you have a plan for what to do in a bushfire."