An uncertain global trade environment and the possibility of an El Nino will keep the local agricultural sector alert despite an otherwise promising outlook.

Millions of Australians will wake to cooler conditions on Wednesday after a day of record-breaking heat, though not everyone will escape the scorching temperatures.

Dubbo recorded its hottest January day on Monday, while on Tuesday Melbourne is expected to suffer through its hottest day since the devastating Black Saturday bushfires with firefighters in multiple states on high alert.

Australians are being warned about the dangers of heat stress as southern states swelter and one city smashes its Australia Day heat record.