Yellow balloons tethered to a bridge brightened up an otherwise dreary day in Auckland in a "random act of fun".

The propagator who attached the balloons to the Newton pedestrian footbridge over the Northwestern Motorway yesterday did not want to be named but said she did it to cheer people up.

The bright yellow balloons swarmed around people who walked across the bridge as the balloons were tossed about in the wind.

One person tweeted that the balloons made them smile as they drove over the Northwestern Motorway and called it a "random act of fun".