The term is used to describe a situation where fires are uncontrollable and fast moving, with evacuation the only safe option.

40 of the fires across New South Wales are uncontained, although there've been no reports of injuries or damage to homes so far.

Daytime temperatures have hit 45 degrees, and have stayed up around 38 degrees in parts right through until midnight.

Many residents in Sydney have taken to the sea to try to cool off.

But these tourists on Bondi Beach don't seem to mind the heat: