This paper presents the first application of pre-stressed carbon fibre-reinforced polymer (CFRP) material for strengthening of a metallic bridge in Australia. The study also describes the application of a wireless sensor network (WSN) system for long-term structural health monitoring (SHM) of the installed retrofit system. A flat pre-stressed un-bonded reinforcement (FPUR) system was developed to apply prestressed CFRP strips to the steel girders of Diamond-Creek Bridge. The bridge is a 121-year-old metallic roadway bridge in Melbourne, which is subjected to daily passenger and heavy truck vehicles. Sets of laboratory tests were performed to examine the efficiency of the proposed FPUR system, prior to its installation on the bridge. Furthermore, in order to demonstrate the efficiency of the proposed retrofit technique, the bridge was instrumented with different types of sensors (including strain gauges, temperature and humidity sensors) and short- and long-term measurements were performed. As for short-term measurements, the bridge was loaded by a 42.5-tonnes semi-trailer before and after strengthening. For the long-term monitoring, a WSN system was used to monitor the prestress level in the CFRP reinforcements for at least one year. The CFRP plates were pre-stressed up to about 980 MPa (≈38% of the CFRP ultimate strength), which resulted in about 50% reduction in the maximum tensile stress in the bridge girders. The results of the short- and long-term measurements in this study showed that the FPUR system was very effective for flexural and fatigue strengthening of the bridge girders.