Mysterious cloud formations puzzle residents - but do you know what caused them?

Sydneysiders were left arching their necks on Wednesday to catch a glimpse of a remarkable cloud formation that experts say was very rare.

Social media was flooded with images of the dark and stormy skyline that rolled through across the city in the afternoon.

“I’ve never seen anything like it,” one local resident wrote.

“What is going on in Sydney’s sky at the moment? [It’s] spectacular!” another wrote.

The incredible formation is referred to as mammatus clouds and are mainly associated with thunderstorms.

Bureau of Meteorology Duty Forecaster Jordan Notora told Yahoo7 News that all the conditions need to align for it to occur.

“These clouds are generally rare and are associated with severe storm activity,” he explained.

He said the mammatus clouds are more likely to occur in summer as a result of pockets of cooler air forming little nodules in the sky, which gives it the stunning pattern.

“Quite light winds aided the smooth shape of the clouds… these conditions can obviously exacerbate the storm activity,” he said.

Despite its menacing appearance, the brutal storm that many Sydneysiders were expecting never really hit the city.

It did however pummel other parts of the state, with the the Blue Mountains and Southern Highlands battered with hail and heavy rain.