Figures released on Friday showed 80,938 cars, trucks and buses were sold in September, some 3.1 per cent down on the corresponding month last year.
Sales in most passenger car and SUV segments were down, except for small and medium cars, according to the Federal Chamber of Automotive Industries.
"The September figures provide further evidence that car buyers are cautious about the current economic situation and outlook," Chamber chief executive Andrew McKellar said on Friday in a statement.
"The trend in sales over recent months provides justification for further action to cut interest rates as soon as possible.
"The industry is looking to rebuild confidence in the marketplace and decisive action to take further pressure off interest rates will provide some welcome relief to motorists."
Small car sales rose 4.1 per cent last month compared to September last year, while medium car sales increased one per cent.
On a year to date basis, new vehicle sales were up 0.3 per cent compared to the same period last year with a total of 784,932 vehicles sold this year.
Toyota retained the top sales position in September with a market share of 22.4 per cent, followed by Holden with 13.1 per cent and Ford with 11 per cent.
So far this year, Toyota has sold 183,806 vehicles, Holden 99,600 and Ford 81,416.
