A full-scale fire test was conducted to investigate the performance of a high-pressure water mist system in the Runehamar Test Tunnel in 2021 in Norway. The tunnel is approximately 1.6 km long, 9 m wide, and 6 m high. The main fire load consisted of 408 standardised wood pallets to represent a Class A fire of Heavy Goods Vehicles (HGV). The activation of the water mist system was determined by the gas temperature measurement above the mock-up, with a threshold value of 60 °C. The water mist pump unit was activated manually with a time delay of 3 min. It was found that the HRR (heat release rate) was reduced to 44 MW in the simulated HGV fire designed according to SOLIT2 guidelines. Moreover, the system was able to prevent fire spread to a target placed 5 m away from the fire source. The tests conducted were based on SOLIT² guidelines but with minor modifications. Key measurements, such as gas temperature, gas concentrations, heat release rate and heat flux, are discussed. The guidelines for performing tunnel fire tests with water suppression systems are discussed, and recommendations are provided.