The northern slopes, including Gunnedah, Tamworth, and the Liverpool Plains, has a very high rating today, which will increase to an extreme rating on Tuesday.

A state of emergency has also been declared by NSW premier Gladys Berejiklian as bushfires rage across the state and rating levels in the greater Sydney and greater Hunter areas rise to catastrophic.

Tamworth and Region Weather's David Farrenden said in a Facebook post the increased ratings were put in place due to hot and windy weather.

"Warm to hot temperatures, mixed with dry strong NW winds, low humidity will result in extreme fire ratings for the NW slopes and plains," he posted.

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According to the Bureau of Meteorology, temperatures are expected to reach 37 degrees in Gunnedah, 35 in Tamworth, and 36 in Quirindi.

Winds are set to be 20-30km/h in the day to 20-45km/h in the afternoon. Gusts in elevated areas could exceed 80km/h.

There may also be raised dust as a result of the windy conditions.

Mr Farrenden urged his followers to stay safe and ensure they have a bushfire plan.

"If you live in a bushy area you need to pay special attention of what's going on around you. Even if there are no fires near you, fires under these conditions will start quickly and spread easily," he wrote.

"Already a massive amount of fires [are] burning throughout the state, you need keep up to date with the latest fire warnings via [the app] Fires Near Me, as conditions will change very quickly.

"Activate your bush fire plan and pack you car if you need to go at short notice. Your safest option is to leave early. This very much could be a life threatening situation and a serious threat to property."

When a fire is burning under an extreme rating, the NSW Rural Fire Service (RFS) suggests leaving early is the safest option for survival and residents should only consider staying if they are prepared to the highest level.

This means the individual's house is specially designed, constructed or modified, and situated to withstand a fire, and they are well prepared and can actively defend their house if a fire starts.

Everyone is urged to keep the Fires Near Me app on hand and ensure their bushfire plan is ready to go. Report unattended fires to Triple Zero (000) immediately.

Further information can be sourced using the using the NSW RFS website or by calling the NSW RFS Bush Fire Information Line on 1800 679 737.