In NSW, two fires were causing concern as thousands of firefighters remained on standby across the state.

In South Australia, a major blaze destroyed more than 600ha of stubble and grassland near Curramulka on Yorke Peninsula and continues to burn fiercely.

Five firefighters were hurt when two fire trucks collided in dense smoke.

A second South Australian blaze was burning near Streaky Bay on the state's west coast along with a series of smaller fires sparked by lightning strikes as thunderstorms swept into the state.

In NSW, the Rural Fire Service said a blaze that broke out at Thunderbolts Way, near Inverell, on Tuesday had blackened more than 2000ha and another, at Sawn Rocks, north-east of Narrabri, more than 200ha.

Fire crews were also strengthening containment lines at a fire that had burned 850ha of scrub and bushland in the Kerringle State Forest, 60km south of Narrabri.

Crews were also mopping up and fortifying containment lines at Mount Canobolas, in the central west, where a blaze had blackened about 80ha.

With the state baking in 40-plus temperatures, fire service commissioner Shane Fitzsimmons warned it would be a difficult fire season.

''Ninety-five per cent of NSW is either officially drought-declared or drought-affected ... so when fires do start they burn quickly, spread rapidly and containment is very difficult,'' he said.

He urged residents living in bushfire-prone areas to be vigilant and prepare homes and families.

The horror conditions across South Australia yesterday prompted authorities to declare a catastrophic fire risk in three districts, the west coast, the eastern Eyre Peninsula and the lower Eyre Peninsula.

Several schools were closed in those areas and police launched an operation to monitor the movements of known firebugs.

In Victoria, the state escaped largely unscathed from its most threatening fire conditions since February's Black Saturday.

Firefighters were called to a number of small grass fires across the state but none got out of control, a Country Fire Authority spokesman said.

After predictions that yesterday's conditions would be the worst in the state since the February 7 fires that killed 173 people, firefighters were relieved when temperatures were a few degrees lower than expected. AAP