Brisbane has reached 40 degrees for the first time in more than a decade during its equal hottest December day on record.

Hot westerly winds ahead of an approaching cool change cased the mercury to soar in Brisbane on Monday.

The city had already exceeded 40 degrees by 1pm and was up to 41.2 degrees by 1:40pm. This is Brisbane's highest temperature since 41.7 degrees in 2004. It's also the city's equal hottest December day since 1981.

A cooler southeasterly change will sweep over Brisbane this afternoon, flushing the heat out of the city. Tuesday is only forecast to reach a top of 30 degrees.

Brisbane's hottest day on record was 43.2 degrees in January 1940.