In this paper, a new developed wireless sensor unit is introduced and utilised to extract dynamic characteristics of a post-tensioned concrete bridge in New Zealand. The system includes 20 wireless senor nodes and one base station unit. The sensor nodes use wireless mesh network to transfer the measurements including temperature, humidity and 3-axis acceleration. The advantages of the sensor nodes are its high resolution and sensitivity, low cost and power consumption to record both ambient and earthquake-induced vibrations using two time-triggered and event- triggered modes. To assess the condition of the superstructure over time, the bridge dynamic characteristics obtained using the vibration recorded from the structure are compared with the counterparts measured several years ago using standalone MEMS accelerometers. The dynamic characteristics of the bridge show a constant performance of the full-scale structure over its lifetime. Also, the results indicate a reliable performance of the developed wireless sensor system for monitoring of large-scale structures.