Australia looks set to sweat through one of its hottest Decembers ever, the national weather bureau said on Friday, prompting fire bans, health warnings and big crowds trying to cool off at beaches.

As the country's hottest town, Marble Bar in the remote northwest, endured its warmest day ever, forecasters said the heat would spread southeast where the big cities of Sydney, Melbourne and Adelaide would endure monthly average temperatures up to 16 degrees Celsius (29 degrees Fahrenheit) higher than usual.

The capital, Canberra, was bracing for its hottest December day on record — 39 C (102 F) — on Saturday.