An ominous-looking shelf cloud has darkened the skies above Sydney, but it won't bring too much of a storm with it, the weather bureau says.

Instagrammers whipped their phones out about 3pm on Thursday to capture the sight of the storm clouds hovering over the city.

"It looks ominous because it’s right over us, and it’s hidden the sun away, but it should clear within the next hour or so," Mohammed Nadi, senior forecaster at the Bureau of Meteorology, said.

The cloud will bring a decent shower and a few rumbles of thunder, but nothing too serious, he said.

"Once this clears, we’ll have some cloud left about but we’re not going to get too many showers especially into tomorrow."