This study aimed to evaluate the effectiveness of three proposed modifications to the structural characteristics of electronic gaming machines as harm minimisation strategies for non-problem and probable problem gamblers. Structural changes included reducing the maximum bet size, reducing reel spin and removing large note acceptors. Behavioural patterns of play were observed in 779 participants attending clubs and hotels. Observations were conducted in the gaming venue during regular gaming sessions. Eight experimental machines were designed to represent every combination of the modifications. 210 participants played at least one modified and one unmodified machine. Following play, the South Oaks Gambling Screen (SOGS) was administered. More problem than non-problem gamblers used high denomination bill acceptors and bet over one-dollar per wager. Machines modified to accept the one-dollar maximum bet were played for less time and were associated with smaller losses, fewer individual wagers and lower levels of alcohol consumption and smoking. It was concluded that the reduction of maximum bet levels was the only modification likely to be effective as a harm minimization strategy for problem gamblers.