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.
It would include using "explosive demolitions that may be heard by members of the public residing in the general area", the letter said.
Feedback on social media is that the large bangs were heard from as far south as Waiuku and as far north as Otara, with some reporting their houses shook.
There have also been comments from residents in Papakura, Manurewa and Takanini wondering what the bangs were at 6.25pm.
Grace Norton commented on the Papakura Spread the News Facebook page that it was "quick" noise and "sounded more like a bomb" rather than thunder.
Clara McKinlay posted on the page: "We heard the two loud bangs and it shook our house, and we're out in Clevedon."
Northern fire communications shift manager Jaron Philips said the fire service had received one call from an Otara resident who said they heard a large explosion which rattled their windows.