September new car sales in New Zealand were "certainly heartening" but the vehicle market was still vulnerable to current economic conditions, a local trade group said on Tuesday.

According to the Motor Industry Association (MIA), September's sales tally of 5,593 new passenger cars was the highest for a month so far in 2009 and up 21% on August, though down 16% year on year.

Although down 21% year on year, commercial vehicle sales of 1,453 were up 25% over August.

"It is encouraging to see that September was the highest month for passenger sales in 11 months," said MIA CEO Perry Kerr. "The pick-up in the economy is starting to flow through to vehicle sales. However, we remain cautious until we see more of a definitive and sustained increase in sales figures."

Toyota, Ford and Holden remained the top three passenger car brands while Toyota, Ford and Mitsubishi topped the commercial chart.  Year to date, Toyota leads with overall total market share of 20.15%, ahead of  Ford (12.35%) and GM's Holden (9.25%).

Toyota's Corolla was the top selling passenger model last month, followed by the Ford Falcon and Holden Commodore. Year to date, the Corolla remained the country's best-selling model with sales of 3,034 units ahead of the Holden Commodore (2,145) and Suzuki Swift (1,804).