The Bureau of Meteorology has warned of possible flash flooding in the Queanbeyan and Molonglo rivers on Friday.

Brisbane's sweaty masses are set for a slight reprieve leading into the weekend, with the high temperatures and sticky humidity both set to become that little more bearable.

Two major industry groups are asking the Australian and Queensland governments to financially support flood mitigation projects in the Greater Brisbane region that have been proven to work since the 2011 floods.

If Thursday hits its predicted peak it will be the ACT's longest March heat spell on record.

Global temperature records continue to tumble, with August easily the hottest in 124 years of data, Japan's Meteorological Agency says.