High petrol prices are being credited with helping to reduce the number of vehicles on roads in the main centres.
The New Zealand Transport Agency says average daily traffic dropped between 5% and 9% in August.
Traffic across the Auckland harbour bridge was also down more than 10% for July and August, compared with the same time last year.
NZTA network operations manager Roly Frost says more motorists are taking public transport or using their cars less, particularly on weekends, due to higher petrol costs.
Green Party co-leader Jeanette Fitzsimons says it is encouraging to see commuters using their vehicles less, but more needs to be done to meet a growing demand for public transport.