Some people thought it was an earthquake or a truck crash; others, quarrying explosions or even a bomb.
People around South Auckland reported hearing two large explosions in the area tonight, with the bangs shaking houses and rattling windows.
Emergency services were initially stumped tonight as to the cause of the loud noises.
But police have now confirmed there was a training exercise involving explosives at the Otahuhu Power Station, which is likely to have been the source of the bangs.
Local residents received letters warning police would be training at the power station between 4pm and 7pm this evening.