The average amount that consumers are spending on a debit card transaction is going down - a trend attributed to the increasing use of contactless payment cards.According to the latest Reserve Bank debit card statistics, consumers made 317.9 million purchases using debit cards in July. Those purchases were worth $17.6 billion - an average of  $55.63 per transaction. Twelve months ago the average was $56.71 per transaction, and two years ago it was $57.74 per transaction.RFi Consulting managing director Lance Blockley said consumers had a threshold for the minimum they were prepared to spend on a card. When using a traditional card, for most people that amount is around $50 or $60.With a contactless card, the minimum that most people are prepared to spend is closer to $30. Blockley said one of the main reasons people continue to use cash is that they think it is the most suitable payment instrument for small amounts ($20 or less). Contactless cards are bringing the minimum card transaction threshold down.Blockley said: "Contactless is helping displace cash. Cash withdrawal from ATMs have been trending down since 2008, while the number of debt card purchases per head has been going up."