The pilot and flight instructor were practicing straight-in, full touch down power off landings (autorotations) in the helicopter at night, with the aid of night vision goggles (NVGs). The pilot determined where on the runway he was going to land and entered the autorotation at an approximate airspeed of 60 knots. When the helicopter was about 90 to 100 ft above the ground, and the engine rpm was at 99-100%, he entered the flare. At the bottom of the flare, the pilot “bumped” up the collective to arrest the rate of descent and leveled out at what he thought was about 7-10 ft above the runway. At that point, the pilot said it felt like the bottom of the helicopter started to fall out from underneath him. He added more collective to arrest the sink rate, but it had no impact on stopping the vertical sink rate. The low rotor rpm horn sounded right before the helicopter landed hard on the runway. The impact was sufficient to flex the main rotor blades and sever the tail boom. The main rotor blades were also substantially damaged. . The operator reported there were no mechanical deficiencies of the helicopter that contributed to the accident. The runway had been recently paved and was very dark in color. The pilot had conducted “hundreds” of NVG power off landings to this runway in the past, but this was his first time after the runway had been re-paved. This was the flight instructor’s first time executing a practice NVG power off landing to this runway. As such, the visual cues that the pilot was used to experiencing had changed, and most likely affected his ability to judge the helicopter’s height and speed above the ground, which resulted in a higher-than-normal flair, low rotor rpm, and subsequent hard landing.