NSW residents are being urged to keep safe as a heatwave grips most of the state, while a total fire ban has been declared.

The mercury is tipped to reach near or above 40C in many parts of the state on Wednesday, with temperatures in the outback town of Bourke forecast to hit 45C.

Residents in the town of Tibooburra, also in the state’s north-west, sweltered overnight with a minimum temperature of 31.2C.

Sydneysiders fared better, but many still had a hot night with temperatures only dropping to 23C in Penrith and 24C at Observatory Hill near the Harbour Bridge, according to the weather bureau.

Most of the city will reach 38C during the day, but some western suburbs, including Penrith and Liverpool, are expected to nudge over 40C.

A total fire ban has been declared for much of the state, including the greater Sydney and greater Hunter regions due to stronger winds and higher temperatures.

Vegetation has dried out considerably, particularly in western NSW, and there’s low humidity and winds are up, said an RFS spokesman.

NSW Health is also asking people to phone their elderly friends, neighbours and relatives at least once a day and eat smaller cold meals such as salad and fruit.

“The best way to reduce the risk of heat-related illness is to drink plenty of water and keep your body as cool as possible, and avoid prolonged sun exposure by staying indoors in cool or air-conditioned facilities,” the Bureau of Meteorology (BOM) said in a statement.

Surf lifesavers are urging those who head to the beach to watch themselves and loved ones for signs of heat stress, such as dizziness, fatigue and headaches.

They also want swimmers to stay between the flags.

The mercury is tipped to keep rising across the state with the Bureau predicting a much warmer day on Friday with mid-to-high 40-degree temperatures in the west of the state.

Bourke is expected to reach a blistering 47C.

For NSW residents keen to escape the heat, the Alpine villages of Thredbo and Perisher will be the coolest spots, with a forecast temperature range of 7C to 22C on Wednesday.