A fire that gutted the post office building in the Johannesburg central business district was put out on Monday morning, emergency services said.

"Fire safety officers will now visit the scene to determine the cause [of the fire]," said spokesperson Nana Radebe.

As many as 30 fire-fighters worked to extinguish the blaze, which started around 8.50pm on Sunday.

Radebe said the building, across the street from the Gauteng Legislature, was currently under construction. This made it difficult for fire-fighters enter it.

The fire was finally extinguished around 4am on Monday.

No one was injured in the fire, said Radebe.