Tuesday's figures from Australia and New Zealand Banking Group showed total job ads slid 6.7% in December, from November when they fell 1.8%.
Job ads averaged 142,569 in December, down 18.8% from a year earlier and the lowest reading since April 2016.
"The number of job ads declined by more than we would expect for that time of the year, suggesting that the escalating bushfire crises had an impact," said Catherine Birch, a senior economist at ANZ.
"Based on previous major natural disasters, such as Victoria's Black Saturday fires and Queensland's 2010-11 floods, the current bushfires could see a short-term negative impact on employment."
Fires have been sweeping across the eastern and southern coast of Australia, scorching rural towns and forcing mass evacuations.
The ANZ vacancies series is closely watched by the Reserve Bank of Australia (RBA) given it counts actual job ads, while Australian Bureau of Statistics data is based on more tenuous intentions by firms to hire.
The RBA has cut its cash rate to a record low of 0.75% in recent months in part as a response to a rising unemployment rate.
Official data showed employment bounced by a solid 39,900 in November, after a shock drop the month before, while the jobless rate eased back to 5.2%. The December data are due on Jan. 23.
(Reporting by Wayne Cole; Editing by Sam Holmes)